Physics Laboratory Manual
PHYC 10180
Physics for Ag. Science
2016-2017
Name.................................................................................
Partner’s Name ................................................................
Demonstrator ...................................................................
Group ...............................................................................
Laboratory Time ...............................................................
Contents
3
Introduction
Laboratory Schedule
4
5
Grading Process and Lab Rules
UCD plagiarism statement
Experiments: Springs
6
7
Investigation into the Behaviour of Gases
and a Determination of Absolute Zero
15
Newton’s Second Law
25
Heat Capactity
35
An Investigation of Fluid Flow using a Venturi
Apparatus
41
Archimedes’ Principle and Experimental
Measurements
47
Rotation
53
Graphing
63
2
Introduction
Physics is an experimental science. The theory that is presented in lectures has its
origins in, and is validated by, experiment.
Laboratories are staged through the semester in parallel to the lectures. They serve a
number of purposes:



an opportunity, as a scientist, to test theories by conducting meaningful
scientific experiments;
a means to enrich and deepen understanding of physical concepts
presented in lectures;
an opportunity to develop experimental techniques, in particular skills
of data analysis, the understanding of experimental uncertainty, and the
development of graphical visualisation of data.
Based on these skills, you are expected to present experimental results in a logical
fashion (graphically and in calculations), to use units correctly and consistently, and to
plot graphs with appropriate axis labels and scales. You will have to draw clear
conclusions (as briefly as possible) from the experimental investigations, on what are
the major findings of each experiment and whetever or not they are consistent with your
predictions. You should also demonstrate an appreciation of the concept of
experimental uncertainty and estimate its impact on the final result.
Some of the experiments in the manual may appear similar to those at school, but the
emphasis and expectations are likely to be different. Do not treat this manual as a
‘cooking recipe’ where you follow a prescription. Instead, understand what it is you are
doing, why you are asked to plot certain quantities, and how experimental uncertainties
affect your results. It is more important to understand and show your understanding
in the write-ups than it is to rush through each experiment ticking the boxes.
This manual includes blanks for entering most of your observations. Additional space is
included at the end of each experiment for other relevant information. All data,
observations and conclusions should be entered in this manual. Graphs may be
produced by hand or electronically (details of a simple computer package are provided)
and should be secured to this manual.
There will be six 2-hour practical laboratories in this module evaluated by continual
assessment. Note that each laboratory is worth 5% so each laboratory session makes
a significant contribution to your final mark for the module. Consequently, attendance
and application during the laboratories are of the utmost importance. At the end of each
laboratory session, your demonstrator will collect your work and mark it.
3
Laboratory Schedule
Please consult the notice boards in the School of Physics, Blackboard, or contact the
lab manager, Thomas O’Reilly (Science East Room 1.41) to see which of the
experiments you will be performing each week. This information is also summarized
below.
Timetable:
Wednesday 2-4: Groups 1,3,5, and 7
Wednesday 4-6: Groups 2,4,8,10 and 11
Friday 3-5: Groups 6 and 9.
Dates
Semester
Week
12-16 Sept
1
19-23 Sept
2
26-30 Sept
3
3–7
Oct
10-14
Oct
17-21 Oct
4
24-28 Oct
7
31 Oct – 4 Nov
8
7-11
Nov
14-18 Nov
9
21-25
Nov
28 Nov – 2 Dec
5
6
10
11
12
Science East 143
Springs:
1,2
Springs:
7,8
Gas:
1,2
Gas:
7,8
Gas:
5,6
Gas:
11
Gas:
3,4
Gas:
9,10
Rotation:
1,2
Rotation:
7,8
Rotation:
5,6
Rotation:
11
4
Room
Science East 144
Springs:
3,4
Springs:
9,10
Newton 2:
3,4
Newton 2:
9,10
Newton 2:
1,2
Newton 2:
7,8
Newton 2:
5,6
Newton 2:
11
Archimedes:
3,4
Archimedes:
9,10
Archimedes:
1,2
Archimedes:
7,8
Science East 145
Springs:
5,6
Springs:
11
Heat Capacity:
5,6
Heat Capacity:
11
Heat Capacity:
3,4
Heat Capacity:
9,10
Heat Capacity:
1,2
Heat Capacity:
7,8
Fluids:
5,6
Fluids:
11
Fluids:
3,4
Fluids:
9,10
Grading Process
Grading is an important feedback for students and as such this is staged through the
semester in close synchronisation with the labs. Students benefit from this feedback for
continuous improvement through the semester. This is the grading process:
1.
A lab script is graded and returned to the student in the subsequent scheduled lab
slot.
2.
Students resolve concerns regarding their grade with the demonstrator either during
or immediately after this lab slot.
3.
Grades are preliminary but can be expected to count towards a module grade once
visible online. A grade is visible online within two weeks of the graded script being
returned.
If the above doesn’t happen, it is the student responsibility to resolve this with the
demonstrator as early as possible.
Lab Rules
1.
No eating or drinking
2.
Bags and belongings should be placed on the shelves provided in the labs
3.
Students must have their lab manual with them and bring any completed pre-lab
assignments to the lab
4.
It is the student’s responsibility to attend an originally assigned lab slot. Zero grade
is assigned by default for no attendance at this lab.
In the case of unavoidable absence, it is the student’s responsibility to complete the
lab in an alternative slot as soon as possible. The student is graded if doing so,
however such a slot can’t be guaranteed as lab numbers are strictly limited. The lab
manager, Thomas O’Reilly (Room Science East 1.41), may be of help in discussing
potential alternative lab times. Where best efforts have been made to attend an
alternative but this still hasn’t been possible, students should then discuss with their
module coordinator.
5.
Students work in pairs in the lab, however students are reminded that reports should
be prepared individually and should comply with UCD plagiarism policy (see next
page).
5
UCD Plagiarism Statement
(taken from http://www.ucd.ie/registry/academicsecretariat/docs/plagiarism_po.pdf)
The creation of knowledge and wider understanding in all academic disciplines depends
on building from existing sources of knowledge. The University upholds the principle of
academic integrity, whereby appropriate acknowledgement is given to the contributions
of others in any work, through appropriate internal citations and references. Students
should be aware that good referencing is integral to the study of any subject and part of
good academic practice.
The University understands plagiarism to be the inclusion of another person’s
writings or ideas or works, in any formally presented work (including essays,
theses, projects, laboratory reports, examinations, oral, poster or slide
presentations) which form part of the assessment requirements for a module or
programme of study, without due acknowledgement either wholly or in part of the
original source of the material through appropriate citation. Plagiarism is a form of
academic dishonesty, where ideas are presented falsely, either implicitly or explicitly, as
being the original thought of the author’s. The presentation of work, which contains the
ideas, or work of others without appropriate attribution and citation, (other than
information that can be generally accepted to be common knowledge which is generally
known and does not require to be formally cited in a written piece of work) is an act of
plagiarism. It can include the following:
1. Presenting work authored by a third party, including other students, friends,
family, or work purchased through internet services;
2. Presenting work copied extensively with only minor textual changes from the
internet, books, journals or any other source;
3. Improper paraphrasing, where a passage or idea is summarised without due
acknowledgement of the original source;
4. Failing to include citation of all original sources;
5. Representing collaborative work as one’s own;
Plagiarism is a serious academic offence. While plagiarism may be easy to commit
unintentionally, it is defined by the act not the intention. All students are responsible for
being familiar with the University’s policy statement on plagiarism and are encouraged,
if in doubt, to seek guidance from an academic member of staff. The University
advocates a developmental approach to plagiarism and encourages students to adopt
good academic practice by maintaining academic integrity in the presentation of all
academic work.
6
Name:___________________________
Student No: ________________
Partner:____________________Date:___________Demonstrator:________________
Springs
What should I expect in this experiment?
You will investigate how springs stretch when different objects are attached to them and
learn about graphing scientific data.
Introduction: Plotting Scientific Data
In many scientific disciplines, and
particularly in physics, you will often
come across plots similar to those shown
here.
Note some common features:
 Horizontal and vertical axes;
 Axes have labels and units;
 Axes have a scale;
 Points with a short horizontal
and/or vertical line through them;
 A curve or line superimposed.
Why do we make such plots?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
7
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
What features of the graphs do you think are important and why?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
The equation of a straight line is often written as
y = m x + c.
y tells you how far up the point is, whilst x is how
far along the x-axis the point is.
m is the slope (gradient) of the line, it tells you how
steep the line is and is calculated by dividing the
change in y by the change is x, for the same part
of the line. A large value of m, indicates a steep
line with a large change in y for a given change in
x.
c is the intercept of the line and is the point where
the line crosses the y-axis, at x = 0.
8
Apparatus
In this experiment you will use 2 different
springs, a retort stand, a ruler, a mass hanger
and a number of different masses.
Preparation
Set up the experiment as indicated in the
photographs. Use one of the two springs (call
this one spring 1). Determine the initial length
of the spring, be careful to pick two reference
points that define the length of the spring
before you attach any mass or the mass
hanger to the spring. Record the value of the
length of the spring below.
Think about how precisely you can determine the length of the spring. Repeat the
procedure for spring 2.
Calculate the initial length of spring 1.
______________________________________________________________________
Calculate the initial length of spring 2.
______________________________________________________________________
Procedure
Attach spring 1 again and then attach the mass hanger and 20g onto the spring.
Measure the new length of the spring, using the same two reference points that you
used to determine the initial spring length.
Length of springs with mass hanger attached (+ 20 g)
Spring 1
Position of the top of spring
Position of the bottom of spring
New length of spring
9
Spring 2
Add another 10 gram disk to the mass hanger, determine the new length of the spring.
Complete the table below.
Measurements for spring 1
Object Added
Disk 1
Disk 2
Disk 3
Disk 4
Disk 5
Total mass
added to the
spring (g)
20
30
New reference
position (cm)
New spring
length (cm)
Add more mass disks, one-by-one, to the mass hanger and record the new lengths in
the table above. In the column labelled ‘total mass added to the mass hanger’, calculate
the total mass added due to the disks.
Carry out the same procedure for spring 2 and complete the table below. Use the same
number of weights as for spring 1.
Measurements for spring 2
Object Added
Disk 1
Disk 2
Total mass
added to the
spring (g)
20
New reference
position (cm)
New spring
length (cm)
Graphical Analysis
Add the data you have gathered for both springs to the graph below. You should plot
the length of the spring in centimetres on the vertical y-axis and the total mass added to
the hanger in grams on the horizontal axis. Start the y-axis at -10 cm and have the xaxis run from -40 to 100 g. Choose a scale that is simple to read and expands the data
so it is spread across the page. Label your axes and include other features of the graph
that you consider important.
10
Graph representing the length of the two springs for different masses added to the
hanger.
11
In everyday language we may use the word ‘slope’ to describe a property of a hill. For
example, we may say that a hill has a steep or gentle slope. Generally, the slope tells
you how much the value on the vertical axis changes for a given change on the x-axis.
Algebraically a straight line can be described by y = mx + c where x and y refer to
any data on the x and y axes respectively, m is the slope of the line (change in
y/change in x), and c is the intercept (where the line crosses the y-axis at x=0).
Suppose the data should be consistent with straight lines. Superimpose the two best
straight lines, one for each spring, that you can draw on the data points.
Examine the graphs you have drawn and describe the ‘steepness’ of the slopes for
spring 1 and spring 2.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Work out the slopes of the two lines.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
What are the two intercepts?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
How can you use the slopes of the two lines to compare the stiffness of the springs?
______________________________________________________________________
______________________________________________________________________
12
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
In this experiment the points at which the straight lines cross the y-axis where x = 0 g
(intercepts) correspond to physical quantities that you have determined in the
experiment. How well do the graphical and measured values compare with each other?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Look at the graphs on page 7, the horizontal and vertical lines through the data points
represent the experimental uncertainty, or precision of measurement in many cases. In
this experiment the uncertainty on the masses is insignificant, whilst those on the length
measurements are related to how accurately the lengths of the springs can be
determined.
How large a vertical line would you consider reasonable for your length measurements?
______________________________________________________________________
What factors might explain any disagreement between the graphically determined
intercepts and the measurements of the corresponding physical quantity?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
The behaviour of the springs in this experiment is said to be “linear”. Looking at your
graphs does this make sense?
______________________________________________________________________
13
14
Name:___________________________
Student No: ________________
Partner:____________________Date:___________Demonstrator:________________
Investigation into the Behaviour of Gases
and a Determination of Absolute Zero
What should I expect in this experiment?
You will investigate gases behaviour, by varying their pressure, temperature and
volume. You will also determine a value of absolute zero.
Pre-lab assignment
If quantity A is inversely proportional to quantity B, what two quantities could you plot to
get a straight line graph?
Define zero on the centigrade temperature scale.
Introduction:
The gas laws are a macroscopic description of the behaviour of gases in terms of
temperature, T, pressure, P, and volume, V. They are a reflection of the microscopic
behaviour of the individual atoms and molecules that make up the gas which move
randomly and collide many billions of times each second.
Their speed is related to the temperature of the gas through the average value of the
kinetic energy. An increase in temperature causes the atoms and molecules to move
faster in the gas. This idea is encapsulated in the equation: ̅̅̅̅̅̅̅̅̅̅̅̅
1/2 𝑚 𝑣 2 = 3/2 k T.
The rate at which the atoms hit the walls of the container is related to the pressure of
the gas.
The volume of the gas is limited by the container it is in.
Using the above description to explain the behaviour of gases, state whether each of
P,V,T goes up (↑), down (↓) or stays the same (0) for the following cases.
P
V
T
A sealed pot containing water
vapour is placed on a hot oven hob.
Blocking the air outlet, a bicycle
pump is depressed.
A balloon full of air is squashed.
A balloon heats up in the sun.
From the arguments above you can see that at constant temperature, decreasing the
volume in which you contain a gas should increase the pressure by a proportional
15
amount.
PV  k .
Thus P  1V or introducing k, a constant of proportionality, P  k V or
This is Boyle’s Law.
Similarly you can see that at constant volume, increasing the temperature should
increase the pressure by a proportional amount. So P  T or introducing  , a
constant of proportionality,
P  T
or
P  .
T
This is Gay Lussac’s Law.
These laws can be combined into the Ideal Gas Law that relates all three quantities by
PV  nRT where n is the number of moles of gas and R is the ideal gas constant. In
these laws the temperature is measured in Kelvin.
Experiment 1: To test the validity of Boyle’s Law
The equipment for testing Boyle’s Law consists of a horizontally mounted tube with a
graduated scale marked along one side to indicate the volume. The scale is either ml or
cl, depending on which apparatus you are using. The tube is sealed by a standard gas
tap next to which is mounted a standard pressure gauge. The effective length of the air
column inside the tube is controlled by altering the position of an inner rubber stopper
moved via a screw thread. The volume of the air column can be read off the scale and
the corresponding pressure taken from the pressure gauge.
Procedure






Ensure that the gas tap is in its open position (i.e. parallel to the tube).
Move the rubber stopper (located inside the tube) to at least 200 ml (20 cl) on the graded
scale by unscrewing the thread knob (anticlockwise movement).
Close the gas tap by turning it 90o clockwise compared to the “open” position.
Observe that initially the pressure reading is at approximately 1.0x105 Pa, which is
atmospheric pressure, or zero, again depending on which apparatus you have.
in the next steps you will increase the pressure of the air column inside the tube upto the
maximum pressure written on the pressure gauge.
Start compressing the air inside the tube by screwing in the thread knob (clockwise
direction).
16



At 10 different positions of the screw, take readings of the air pressure and the
corresponding air volume.
For safety reasons do not exceed the maximum pressure on the pressure gauge
scale!
On the volume scale, each volume value is read by matching the position of the left
extremity of the rubber stopper to the graded scale.
Tabulate your results below.
Once you finished taking readings, depressurize the tube to atmospheric pressure by
turning the gas tap 90o anticlockwise.
Air Pressure (Pa)
Volume of air
(m3)
Air Pressure (Pa)
Volume of air
(m3)
Compare Boyle’s Law: P  k V to the straight line formula y=mx+c.
What quantities should you plot on the x-axis and y–axis in order to get a linear
relationship according to Boyle’s Law? Explain and tabulate these values below.
______________________________________________________________________
______________________________________________________________________
17
x-axis=
y-axis=
Use the straight-line equation to work out what the slope of your graph should be equal
to.
_______________________________
Similarly,
what
should
the
intercept
of
the
graph
equal?
_______________________________
Make a plot of those variables that ought to give a straight line, if Boyle’s Law is correct.
Use either the graph paper below or JagFit (see back of manual) and attach your
printed graph below.
18
19
Comment on the linearity of your plot. Have you shown Boyle’s Law to be true? Does
how precisely you can determine the pressures and volumes influence your answers?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
What is the value for the slope determined from
the graph?
What is the value for the intercept determined
from the graph?
Use the Ideal Gas Law, at a temperature of 20oC, to figure out how many moles of air
are trapped in the column. (The universal gas constant R=8.3145 J/mol K)
Number of moles of gas =
20
Experiment 2: To test the validity of Gay Lussac’s Law
The equipment for testing Gay
Lussac’s Law consists of an enclosed
can surrounded by a heating element.
The volume of gas in the can is
constant. A thermocouple measures
the temperature of the gas and a
pressure transducer measures the
pressure.
Procedure
Plug in the transformer to activate the temperature and pressure sensors. Switch on the
multimeters labelled pressure and temperature, by turning the dial to the oC temperature
scale. Connect the power supply to the apparatus and switch on. At the start your gas
should be at room temperature (≈ 20 oC) and pressure (≈ 100 mV). You can directly use
the mV unit of pressure as Nm-2 or Pa. During the course of this experiment the gas
contained within the apparatus will be heated up to a final temperature of ≈75-100oC.
Immediately switch off the power supply.
While the gas is heating, take temperature and pressure readings at regular intervals
(say every 5oC) and record them in the table below.
Temperature
(oC)
Pressure of gas
(Nm-2)
Temperature
(oC)
21
Pressure of gas
(Nm-2)
Gay Lussac’s Law states P = kT and by our earlier arguments heating the gas makes
the atoms move faster which increases the pressure. We need to think a little about our
scales and in particular what ‘zero’ means. Zero pressure would mean no atoms hitting
the sides of the vessel and by the same token, zero temperature would mean the atoms
have no thermal energy and don’t move. This is known as absolute zero.
The zero on the centigrade scale is the point at which water changes to ice and is
clearly nothing to do with absolute zero. So if you are measuring everything in
centigrade you must change Gay Lussac’s Law to read P = k (T -Tzero ) where T is the
temperature measured in Centigrade and Tzero is absolute zero on the centigrade scale.
Compare P = k (T -Tzero ) to the straight line formula y=mx+c.
What should you plot on the x-axis and y–axis in order to get a linear relationship
according to Gay Lussac’s Law?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
According to the equation of the straight line, what
should the slope of your graph be equal to?
Similarly, what should the intercept of the graph
equal?
Use the equation to determine, how you can work out T zero?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
22
Make a plot of P versus T. Use either the graph paper below or JagFit (see back of
manual) and attach your printed graph below.
23
Comment on the linearity of your plot. Have you shown Gay Lussac’s Law to be true?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
From your graph, what is the value for the slope?
Similarly, what is the value for the intercept?
From these values, calculate an experimental value for absolute zero temperature.
Comment on the precision with which you have been able to find absolute zero
Absolute zero =
24
Name:___________________________
Student No: ________________
Partner:____________________Date:___________Demonstrator:________________
Newton’s Second Law.
What should I expect in this experiment?
This experiment has two parts. In the first part you will apply a fixed force, vary the
mass and note how the acceleration changes. In the second part you will measure the
acceleration due to the force of gravity.
Pre-lab assignment
Acting under constant acceleration, a cart starts from rest and travels 1m west in 5s.
What are the acceleration and average velocity of the cart?
Introduction
Newton’s second law states F  ma , a force causes an acceleration and the size of
the acceleration is directly proportional to the size of the force. Furthermore, the
constant of proportionality is mass.
Apparatus
The apparatus used is shown here and
consists of a cart that can travel along a
low friction track. The cart has a mass
of 0.5kg which can be adjusted by the
addition of steel blocks each of mass
0.5kg. String, a pulley and additional
masses allow forces to be applied to
the carts. A PASCO Xplorer GLX, two
photogates, a PASCO angle indicator
and PASport didgital adapter are also
required.
Take care to ensure that the track is
completely level before starting the
experiments.
25
Configuring the Xplorer GLX
In order to conduct the experiment in an accurate fashion, the PASCO Xplorer GLX
must be configured to give you data about the velocity of the cart at two points along the
track and how long it takes the cart to travel between the gates.












Connect the AC adapter to the wall and plug it into the GLX.
Insert the connector for the first photogate into the port marked ‘1’ on the digital adapter
and insert the second into the port marked ‘2’.
Connect the digital adapter to the GLX. A menu should pop up.
Place the photogates at the 30 and 80 centimetre marks on the track.
Use the arrow keys to navigate to ‘Photogate Timing’ and press the
button to select
it.
Using the navigation and select keys, set Photogate spacing to 0.5000m and ensure that
only ‘Velocity In Gate’ and ‘Time Between Gates’ are set to visible.
Press the Home key to navigate to the home screen.
Press F1 or navigate to the Graph section. Once on the Graph screen press F4 to open
the graph menu. Navigate to ‘Two Measurements’ and press select.
Press the select key and navigate to the ‘Y’, which should be visible on the right hand
side of the graph. Press select and choose ‘Time Between Gates’. Time between gates
will now be displayed to the right of the graph. The left axis should already be displaying
“Velocity in Gates”.
You can now record data by pressing
to begin and stop recording data.
Velocity will be displayed as a line and the time between gates will be displayed as both
a single point on the graph and as a numerical value along the right Y axis of the graph.
To read the data from your completed run accurately, press F3 and select ‘Smart Tool’.
Use the navigation buttons to see the precise values for the points on the graph.
Plotting the GLX data:
Use the arrow keys to highlight your data file in the RAM memory and press “F4
files”. Select the “copy file” option and then choose the external USB as the
destination for the file, and press “F1 OK”.
Transfer the USB data stick to the lab computer and create a graph.
26
Investigation 1: To check that force is proportional to acceleration and show the
constant of proportionality to be mass and calculate the acceleration due to gravity.
The apparatus should be set up as in the
picture. Attach one end of the string to
the cart, pass it over the pulley, and add
a 0.012kg mass to the hook.
The weight of the hanging mass is a force, F, that acts on the cart. The whole system
(both the hanging mass mh and the cart mcart) are accelerated. So long as you don’t
change the hanging mass, F will remain constant. You can then change the mass of
the system, M, by adding mass to the cart, noting the change in acceleration, and
testing the relationship F=ma.
If F=ma and you apply a constant force F, what do you expect will happen to the
acceleration as you increase the mass?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
In the configuration you are using, the GLX will give you data regarding the carts
velocity at two points along the track and the times that it passed through each gate.
Acceleration is rate of change of velocity. You can use the equation below, where the
numbers refer to the times (t) and velocities (v) at the two gates, to calculate the
average acceleration experienced by the cart between the gates.
𝑎=
𝑣2−𝑣1
𝑡2−𝑡1
Place different masses on the cart. Using the GLX, fill in the table.
27
mh
(kg)
mcart
(kg)
0.012
0.5
0.012
1.0
0.012
1.5
M=
mh+
mcart
(kg)
v1
(m/s)
v2
(m/s)
t1
(s)
a
(m/s2)
t2
(s)
M.a
(N)
1/a
(m-1s2)
From the numbers in the table what do you conclude about Newton’s second law?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Analysis and determination of the acceleration due to gravity.
If Newton is correct, F  (mh  mcart )  a
You have varied mcart and noted how the acceleration changed so let’s rearrange the
formula so that it is in the familiar linear form y=mx+c. Then you can graph your data
points and work out a slope and intercept which you can interpret in a physical fashion.
F  (mh  mcart )  a 
m
1 1
 mcart  h
a F
F
So if Newton is correct and you plot mcart on the x-axis and 1/a on the y-axis you should
get a straight line.
In terms of the algebraic quantities above,
what should the slope of the graph be equal to?
In terms of the algebraic quantities above,
what should the intercept of the graph equal?
Plot the graph and see if Newton is right. Do you get a straight line?
What value do you get for the slope?
What value do you get for the intercept?
28
Use JagFit to create your graph (see back of manual) and attach your printed graph
below.
29
Now here comes the power of having plotted your results like this. Although we haven’t
bothered working out the force we applied using the hanging weight, a comparison of
the measured slope and intercept with the predicted values will let you work out F.
From the measurement of the slope, the constant force applied can be calculated to be
From the intercept of the graph you can calculate F in a different way. What value do
you get for F based on the intercept?
Using the two values for the force calculated above, determine a best values for the
force, F. What is the uncertainty in this value?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
You’ve shown that F is proportional to a and the constant of proportionality is mass. If
the force is the gravitational force, it will produce an acceleration due to gravity
(usually written g instead of a) so once again a (gravitational) force F is proportional to
acceleration, g. But what is the constant of proportionality?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
So since that force was just caused by the mass mh falling under gravity, F= mhg, you
can calculate the acceleration due to gravity to be
Comment on how this compares to the accepted value for gravity of about 9.8 m/s 2.
How does the accuracy of your measurements affect this comparison?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
30
Investigation 2: Acceleration down an inclined plane.
The apparatus should be set
up as in the picture. Raise one
end of the track using the
wooden block in place of the
elevator. The carts can be
released from rest at the top of
the track.
A cart on a slope will
experience the force of gravity
which causes the cart to
accelerate and roll down the
incline. The acceleration due
to gravity is as shown in the
schematic. The component of
this acceleration parallel to the
inclined surface is, g.sin and
this is the net acceleration of
the cart (when friction is
neglected).
Note the acceleration depends
on the angle of the incline.
Vary the angle of the incline (repeat for at least four different angles) and measure the
acceleration of the cart. In the same way as was done in investigation 1.
You will need to determine the angle of the incline. Summarise your results in this table.
Angle
Sin
(Angle)
v1 (m/s)
v2 (m/s)
31
t1 (s)
t2 (s)
Acceleration
(ms-2)
Since the acceleration a  g sin  , there is a linear dependence on the sine of the
angle. Make a graph with sin  on the x-axis and a on the y-axis.
What value do you expect (theoretically) for the slope?
What value do you expect (theoretically) for the intercept?
What value do you obtain (experimentally) for the slope?
What value do you measure for the intercept?
Comment on your results. How could you improve your measurements of g?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Use JagFit to create your graph (see back of manual) and attach your printed graph
below.
32
33
34
Name:___________________________
Student No: ________________
Partner:____________________Date:___________Demonstrator:________________
To Determine the Specific Heat Capacity of Metals
Introduction
The heat capacity of a material is a measure of the materials ability to absorb heat, or
thermal energy, for a given rise in temperature. As a body becomes hotter, the atoms
and molecules that comprise it begin to move more and more violently. In the case of
solids, the atoms or molecules vibrate about the positions that they occupy in the solid.
As the temperature of the material increases, the vibrations become more energetic,
eventually breaking the bonds that hold the solid together as the solid begins to melt.
The purpose of this experiment is to measure the heat capacity of two metals, copper
and aluminium, by supplying a known quantity of heat and recording the resultant rise in
temperature.
The heat capacity of an object is defined as the energy required to raise the
temperature of the object by 1 K. Clearly the greater the mass of the object the more
heat is required, so a more useful quantity is the Specific Heat Capacity (SHC) of an
object. This is defined as the energy required to raise the temperature of 1 kg of the
material by 1 K. Expressing this mathematically,
Q  mc(T f  Ti )
where Q is the amount of heat required, m is the mass of the object, Ti & Tf are the
initial and final temperatures of the object and c is the SHC measured in units of Joules
per kg per Kelvin (J kg-1 K-1).
Apparatus:
 1 power supply
 1 12V, 50W Immersion heater
 1 1kg block of copper
 1 1kg block of aluminium
 1 Joule-Watt meter plus 10A shunt
 Laboratory thermometer
 Digital multimeter
 Stopwatch
Health and Safety:
Switch off apparatus after completion of readings.
Take care when handling the blocks and the heater elements, they are very hot.
Do not place the metal blocks directly on the laboratory bench.
Be sure to place the metal blocks on an insulating material, such as a piece of
wood, before heating them.
35
Set up:
 Set the power supply to 12V DC.
 Connect cables from the power supply (DC output) to the Joule-Watt meter.
 Connect the immersion heater to the Joule-Watt meter where it says ‘load’.
 Insert the immersion heater into one of the metal blocks.
 Insert a thermometer into the same block.
Figure 1. Specific heat experimental arrangement, be sure to connect the ac output
(yellow) from the power supply to the Joule-Watt meter.
Procedure:
Start with the aluminium block and repeat for the copper one.




Measure the portion of the heater inserted into the block and express this as a
fraction of the total length of the heater (f).
Ensure that the multimeter is attached in series with the heater between the
positive and negative load ports of the Joule-Watt meter as demonstrated in
Figure 2. Set the multimeter to current (A) (to measure a current > 1A). The
select button will also have to be pressed to swich the current
measurement to DC.
Attach a second multimeter in parallel to the DC ports of the power supply and
set it to measure DC voltage (approximately 12V).
From the current (measured through the heater) and the measured voltages you
can work out the average electrical power, P, supplied to the block.
Figure 2. Set up for measuring the current
flowing through the heater.
36








Reset the energy display on the Joule-Watt meter (by pressing the reset button).
Record the initial temperature of the block,T1.
Switch on the power supply and start the stopwatch.
Record the temperature and current in the table below at one minute intervals
for each material, as it is heating.
When the temperature has risen by 20 degrees (or after 20 minutes,
whichever comes first), switch off the power supply unit. Note down the
energy reading on the Joule-Watt meter and the electrical heating time (theat).
The temperature will continue to rise for a short period after switching off. Wait
until it reaches maximum, then record the temperature T2 and the time t.
Stop, zero and restart the stopwatch.
Note the temperature, T3, after the block has cooled for t seconds.
Figure 3: Timing summary.
Record your
measurements of theat,
the energy read from
the Joule-Watt meter,
T2, T3 and time t here.
Aluminium
Measurements
theat (s)
Energy reading (J)
(from J-W meter)
T2 (oC)
t (s)
T3 (oC)
37
Copper
Record your temperatures below
Time:
(mins)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Time:
(mins)
Heating
Temp
(oC)
Heating
Temp
(oC)
Aluminium
Voltage
Current
(V)
(A)
Cooling
Temp (oC)
Copper
Voltage
Current
(V)
(A)
Cooling
Temp (oC)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
38
Plot the temperature of both blocks as a function of time on the same graph and
comment on the graphs.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Cooling Correction:
The block loses heat during the heating cycle. In the absence of these losses it would
have reached a higher final temperature. The uncorrected final temperature is T2. The
corrected final temperature may be approximated as:
T  T3
T2  2
 T4
2
This formula assumes the dominant heat loss mechanism is due to convection by
heated air, neglecting the cooling due to radiation and conduction. The readings should
be recorded as follows:
Al
Cu
Units
Fraction of the heater inserted in the block, f.
Mass of cylinder, m
Kg
Initial temperature, T1
K
Uncorrected final temperature, T2
K
Electrical Heating Time, theat
Sec
Temperature after t sec cooling, T3
Corrected final temperature, T4
K
K
Rise in temperature, (T4-T1)
K
Average Current through the Heater, I.
A
Average Voltage, V.
Average Power, 𝐏 = 𝐕 𝐱 𝐈
12
Energy supplied to heater, J = (P x theat)
39
12
V
W
J
Energy supplied to the block, E = (J  f)
J
Energy as noted from Joule-Watt meter (J2)
J
Energy supplied to the block, E2 = (J2  f)
J
The specific heat capacity, c, may be calculated from conservation of energy. You
should do a calculation based on the energy reading from the Joule-Watt meter and a
second based on the calculated electrical energy supplied.
Electrical energy supplied  increase in heat energy
Thus,
E = mc(T4 – T1)
c
and therefore
Metal
Aluminium
Copper
Theoretical value
J/(kg K)
E
m(T4  T1 )
Joules / ( kg K)
Measured Value
J/(kg K)
(based on J-W energy)
Measured Value
J/(kg K)
(Based on electrical energy)
960
390
How do your results compare with the theoretical values? Comment on any differences
between your measurements and the accepted values? What are the sources of
uncertainty in the experiment.
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
How do your results for the electrical energy supplied to the heater differ from the
reading on the Joule-Watt meter? Give reasons. Did it affect your results significantly?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
NB: There are three mechanisms for heat loss - convection, conduction and radiation.
In the above experiment we assume convection losses only and ignored the possibility
of losses by conduction, (due to contact of the block and bench) and radiation.
40
Name:___________________________
Student No: ________________
Partner:____________________Date:___________Demonstrator:________________
An Investigation of Fluid Flow using a Venturi
Apparatus.
Theory
When a fluid moves through a channel of varying width, as in a Venturi apparatus, the
volumetric flow rate (volume/time) remains constant but the velocity and pressure of the
fluid vary. In the Venturi apparatus there are two different channel widths.
The flow rate, Q, of the fluid through the tube is related to the speed of the fluid (v) and
the cross-sectional area of the pipe (A). This relationship is known as the continuity
equation, and can be expressed as
𝑄 = 𝐴0 𝑣0 = 𝐴𝑣
(Eq.1)
where A0 and v0 refer to the wide part of the tube and A and v to the narrow part. As the
fluid flows from the narrow part of the pipe to the constriction, the speed increases from
v0 to v and the pressure decreases from P0 to P.
If the pressure change is due only
to the velocity change, a simplified
version of Bernoulli’s equation can
be used:
𝜌
𝑃 = 𝑃0 − 2 {𝑣 2 − 𝑣02 }
(Eq. 2)
Here  is the density of water:
1000 kg/m3.
The flow rate also depends on the pressure, P 0, behind the fluid flowing into the
apparatus. In this experiment the pump provides the pressure that makes the fluid flow
through the apparatus. The higher the power to the pump the greater the pressure and
the faster the fluid flows.
In this experiment you will use the Venturi apparatus to investigate fluid flow and verify
that equation 2 holds. You will also investigate how the volatge supplied to the pump
affects the pressure and flow rate.
41
Apparatus
The apparatus consists of a
reservoir, which is a plastic box, of
water connected to a Venturi
apparatus through which the water
can flow into a collecting beaker,
itself placed in a box. The electrical
pump in the box causes the water
flow in the apparatus.
Procedure
Using the spare apparatus in the laboratory, measure the depth of the channel and the
widths of the wide and narrow sections. Calculate the large (A 0) and small (A) crosssections by multiplying the depth by the width for the two sections of the apparatus.
Depth____________ Width(large)______________ Width (narrow) ______________
Area (A0) ______________ Area (A) ______________
.
Before putting any water in the apparatus, connect the Venturi apparatus to the
Quad pressure sensor and GLX data logger.
1. Connect the tubes from the Venturi apperatus to the Quad Pressure Sensor.
Ensure that they are connected in the right order. The one closest to the flow IN to
the apparatus is connected to number 1 etc.
2. Connect the GLX to the AC adapter and power it up.
3. Connect the Quad Pressure sensor to the GLX. A graph screen will appear if this
is done correctly.
4. Navigate to the home screen (Press “home” key) and then navigate to ‘Digits’
and press select. Data from two of the four sensors should be visible.
5. To make the data from all four sensor visible press the F2 key. All sensor input
should now be visible.
6. Make sure that the eight fixing taps on the Venturi apparatus are all just finger
tight. Fill the reservoir so that the pump is fully submerged in the plastic box.
Ensure that the outlet of the apparatus is in the beaker which is in the box.
Turn on the power suppl to the pump and set the voltage to 10V. Remove any air
bubbles by gently tilting the outlet end of the Venturi apparatus upwards.
7. Calibrate the sensors, using the atmospheric pressure reading:
(i)
Press “home” and “F4” to open the sensors screen.
(ii)
Press “F4” again to open the sensors menu.
(iii)
From the menu, select “calibrate” to open the calibrate sensors window.
(iv)
In the first box of the window, select “quad pressure sensor”.
(v)
In the third box of the window, select “calibrate all similar measurements”.
(vi)
In the “calibration type” box, select “1 point offset”.
(vii) Press “F3”, “read pt 1”.
42
(viii)
Press “F1”, “OK”.
Measure the time that it takes for 400 ml (0.4l = 4x10 -4 m3) of water to flow through the
Venturi apparatus. Be sure to read the water level by looking at the bottom of the
meniscus.
When the flow is steady note the pressures on the four sensors. Enter your data in the
table. Repeat the measurements twice more. Be sure to return all the water to the
reservoir (plastic box). Make sure that the apparatus stays free of air bubbles.
Run
P1
P2
P3
P4
Time
/kPa
/kPa
/kPa
/kPa
/s
1
2
3
Average
Calculate the flow rate, Q, volume/time, from the average value of the time taken for
400 ml to flow through the apparatus. 1ml = 1cm3 = 1x10-6 m3.
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
Use equation (1) and the cross-sectional areas that you calculcated to work out the
velocity of the water in the wide (v0) and narrow (v) sections of the apparatus.
v0=
v=
______________________________________________________________
______________________________________________________________
Which is larger v or v0, is this what you expected?
____________________________________________________________________
____________________________________________________________________
If the apparatus was not constricted the pressure at point 2 (P0) is equal to the average
value of P1 and P3. Use you average valies of P1 and P3 to calculate P0:
P0=1/2(P1+P3) _______________________________________________________
____________________________________________________________________
43
Use equation 2 and your values for P0,v0 and v to calculate a value for P, the pressure
in the narrow section of the apparatus. The SI unit for pressure is the Pascal (Pa), 1kPa
= 1000 Pa = 1000 N/m2 = 1000 kg s-2 m-1.
P= _________________________________________________________________
____________________________________________________________________
____________________________________________________________________
How does your value for the pressure in the constriction compare to the measured
values P2 and P4? You might consider how precisely you can determine both the
calculated and measured pressures.
_________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
Next measure the pressure as for five diferent pump supply voltages to the pump. Be
sure to let the flow stabalise before taking your readings. Complete the table below.
Voltage /V
P1 /kPa
P2 /kPa
P3 /kPa
P4 /kPa
Average
of P1 and
P3 /kPa
Average
of P2 and
P4 /kPa
5
6
8
10
12
When you have finished, allow the reservoir to run empty and tilt the Venturi
apparatus to empty water from it. Once the apparatus is empty of water remove
the pressure tubes from the sensor by gently twisting the white plastic collars
that connected the hoses to the sensor (leave the tubes connected to the underside
of the Venturi apparatus). Empty as much water as you can from the apparatus and
then all of the water into the sink in the laboratory.
44
Plot a graph of the two average pressures as a function of pump voltage on the same
graph. Create a graph either manually or using Jagfit (see back of manual) and attach
your printed graph below.
45
Comment on your graph, is it consistent with what you expected to happen?
__________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
What can you conclude from this part of the experiment?
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
46
Name:___________________________
Student No: ________________
Partner:____________________Date:___________Demonstrator:________________
Archimedes’ Principle and Experimental
Measurements.
Introduction
All the technology we take for granted today, from electricity to motor cars, from
television to X-rays, would not have been possible without a fundamental change,
around the time of the Renaissance, to the way people questioned and reflected upon
their world. Before this time, great theories existed about what made up our universe
and the forces at play there. However, these theories were potentially flawed since they
were never tested. As an example, it was accepted that heavy objects fall faster than
light objects – a reasonable theory. However it wasn’t until Galileo 1 performed an
experiment and dropped two rocks from the top of the Leaning Tower of Pisa that the
theory was shown to be false. Scientific knowledge has advanced since then precisely
because of the cycle of theory and experiment. It is essential that every theory or
hypothesis be tested in order to determine its veracity.
Physics is an experimental science. The theory that you study in lectures is derived
from, and tested by experiment. Therefore in order to prove (or disprove!) the theories
you have studied, you will perform various experiments in the practical laboratories.
First though, we have to think a little about what it means to say that your experiment
confirms or rejects the theoretical hypothesis. Let’s suppose you are measuring the
acceleration due to gravity and you know that at sea level theory and previous
experiments have measured a constant value of g=9.81m/s2. Say your experiment
gives a value of g=10 m/s2. Would you claim the theory is wrong? Would you assume
you had done the experiment incorrectly? Or might the two differing values be
compatible?
What do you think?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Experimental Uncertainties
1
Actually, the story is probably apocryphal. However 11 years before Galileo was born, a similar experiment was
published by Benedetti Giambattista in 1553.
47
When you make a scientific measurement there is some ‘true’ value that you are trying
to estimate and your equipment has some intrinsic uncertainty. Thus you can only
estimate the ‘true’ value up to the uncertainty inherent within your method or your
equpiment. Conventionally you write down your measurement followed by the symbol
 , followed by the uncertainty. A surveying company might report their results as
245  5 km, 253  1 km, 254.2  0.1 km. You can interpret the second number as the
‘margin of error’ or the uncertainty on the measurement. If your uncertainties can be
described using a Gaussian distribution, (which is true most of the time), then the true
value lies within one or two units of uncertainty from the measured value. There is only
a 5% chance that the true value is greater than two units of uncertainty away, and a 1%
chance that it is greater than three units.
Errors may be divided into two classes, systematic and random. A systematic error is
one which is constant throughout a set of readings. A random error is one which
varies and which is equally likely to be positive or negative. Random errors are always
present in an experiment and in the absence of systematic errors cause successive
readings to spread about the true value of the quantity. If in addition a systematic error
is present, the spread is not about the true value but about some displaced value.
Estimating the experimental uncertainty is at least as important as getting the central
value, since it determines the range in which the truth lies.
Practical Example:
Now let’s put this to use by making some very simple measurements in the lab. We’re
going to do about the simplest thing possible and measure the volume of a cylinder
using three different techniques. You should compare these techniques and comment
on your results.
Method 1: Using a ruler
The volume of a cylinder is given by πr2h where r is the radius of the cylinder and h its
height.
Measure and write down the height of the cylinder.
Don’t forget to include the uncertainty and the units.
h=

Measure and write down the diameter of the cylinder.
d=

48
Now calculate the radius.
(Think about what happens to the uncertainty)
r=

Calculate the radius squared – with it’s uncertainty!
r2 =

One general way of determining the uncertainty in a quantity f calculated from others is
to use the following method..
 From your measurements, calculate the final result. Call this f , your answer.
 Now move the value of the source up by its uncertainty.
Recalculate the final result. Call this f  .
 The uncertainty on the final result is the difference in these values.
Finally work out the volume. Estimate how well you can determine the volume.
V=

49
Method 2: Using a micrometer screw
This uses the same prescription. However your precision should be a lot better.
Measure and write down the height of the cylinder.
Don’t forget to include the uncertainty and the units.
h=

Measure and write down the diameter of the cylinder.
d=

Now calculate the radius.
r=

Calculate the radius squared – with it’s uncertainty!
(Show your workings)
r2 =
Finally work out the volume.
V=

50

Method 3: Using Archimedes’ Principle
You’ve heard the story about the ‘Eureka’ moment when Archimedes dashed naked
through the streets having realised that an object submerged in water will displace an
equivalent volume of water. You will repeat his experiment (the displacement part at
least) by immersing the cylinder in water and working out the volume of water displaced.
You can find this volume by measuring the mass of water and noting that a volume of
0.001m3 of water has a mass2 of 1kg.
Write down the mass of water displaced.

Calculate the volume of water displaced.

What is the volume of the cylinder?

Discussion and Conclusions.
Summarise your results, writing down the volume of the cylinder as found from each
method.



Comment on how well they agree, taking account of the uncertainties.
_____________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
2
In fact this is how the metric units are related. A litre of liquid is that quantity that fits into a cube of side 0.1m and
a litre of water has a mass of 1kg.
51
Can you think of any systematic uncertainties that should be considered? Can you
estimate their size?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Requote your results including the systematic uncertainties.






What do you think the volume of the cylinder is? and why?

My best estimate of the volume is

because ______________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
52
Name:___________________________
Student No: ________________
Date:_________ Partner:_______________ Demonstrator:______________________
An Investigation of Rotational Motion
What should I expect in this experiment?
This experiment introduces you to some key concepts concerning rotational motion.
These are: torque (), angular acceleration (), angular velocity (), angular
displacement () and moment of inertia (I). They are the rotational analogues of force
(F), acceleration (a), velocity (v), displacement (s) and mass (m), respectively.
Introduction:
The equations of motion with constant acceleration are similar whether for linear or
rotational motion:
Linear
vaverage 
s2  s1
t2  t1
v1  v2
2
v2  v1  at
vaverage 
s2 = v1t + at
Rotational
average 
average 
 2  1
t2  t1
1  2
2
 2  1  t
q 2 = w1t + a t 2
Eq.1
Eq.2
Eq.3
2
2
2
Eq.4
Introduced in the above table is 𝜔, the angular velocity (rad/s) and 𝜶 the angular
acceleration (rad/s2).
Furthermore, just as a force is proportional to acceleration through the relationship
F=ma, a net torque changes the state of a body’s (rotational) motion by causing an
angular acceleration.
  I
(Eq.5)
Where 𝝉 is the torque (Nm) and I is the moment of inertia (kgm2) of the body
experiencing the torque. The body’s moment of inertia is a measure of resistance to this
change in rotational motion, just as mass is a measure of a body’s resistance to change
in linear motion.
The equation
  I
is the rotational equivalent of Newton’s 2nd law
53
F  ma .
You will use two pieces of apparatus to investigate these equations. The first lets you
apply and calculate torque, measure angular acceleration and determine an unknown
moment of inertia, I of a pair of cylindrical weights located at the ends of a bar. The
second apparatus lets you investigate how I depends on the distribution of mass about
the axis of rotation and lets you determine the value of I, already measured in the first
part, by a second method. You can then compare the results you obtained from the two
methods.
Investigation 1: Measuring I from the torque and angular acceleration:
Place cylindrical masses on the bar
at the r = 0.25m position with
respect to the axis of rotation. The
bar is attached to an axle which is
free to rotate.
The masses
attached to the line wound around
this axle are allowed to fall, causing
a torque about the axle
The value of the torque caused by the falling masses is   Fr where F is the weight
of the mass attached to the string and r is the radius of the axle to which it is attached
(The rod to which the string is wrapped around). Calculate the value of .
Mass, m attached to string (kg)
Force, F = m . g (N)
Radius, r of axle (m)
Torque,  = F  r (Nm)
Next you have to calculate the angular velocity, , and the angular acceleration, .
Describe the difference between these two underlined terms.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
54
Wind the string attached to the mass around the axle until the mass is close to the
pulley. Release it and measure the time for the bar to perform the first complete
rotation, the second complete rotation, the third, fourth and fifth rotation. Estimate your
experimental uncertainties.
Number
of
Rotations
0
1
2
3
4
5
 (rad)
Time (s)
0
0
Since you have the angular distance travelled in a given time you can use Eq. 4 to find
the angular acceleration . The total angular displacement after n rotations in Eq. 4 is
denoted by 𝜃2 . The initial angular velocity at time t=0 In Eq. 4 is denoted by 𝜔1. The
total elapsed time since the releasing of the system is denoted in Eq. 4 by t.
Explain how  can be obtained using Eq. 4
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
=
55
Using Eq.1, calculate the average velocity,  average , during each rotation.
If the acceleration is constant, this will be the instantaneous velocity at a time, tmid,
exactly half-way between the start and the end of that rotation.
Can you see this? Explain why it is so.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Enter the values in the table below and make a plot of
 average
on the y-axis against
tmid on the x-axis.
Number
of
Rotation
 average
(rad / s)
tmid (s)
1
2
3
4
5
Since this graph gives the instantaneous velocity at a given time, Eq. 3 can be used to
find the angular acceleration, .
What value do you get for ?
Now you know  and , so work out I from Eq. 5.
56
Use JagFit (see back of manual) to create your graph of
tmid on the x-axis and attach your printed graph below.
57
 average
on the y-axis against
Investigation 2: How I depends on the distribution of mass in a body
Take the metal bar on the bench and roll it between your hands. Now hold it in the
middle and rotate it about its centre so that the ends are moving most. Which is easier?
Which way does the bar have a higher value of I? Why is it different?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Attach the weights and the bar to
the rotational apparatus known as a
torsion axle. This consists of a
vertical axle connected to a spring
which opposes any departure from
the angle of rotational equilibrium.
Note: The apparatus is very
delicate. Please take care not
to strain it.
When you rotate the bar (always taking care to compress the spring, but not to overcompress it) the spring causes a torque about the axis of rotation which acts to restore
the bar to the equilibrium angle. Usually the bar overshoots, causing an oscillation to
occur. This is exactly analogous to the way a mass on a linear spring undergoes simple
harmonic motion. The period of the oscillations, T, is determined by the restoring torque
in the spring, D, and the moment of inertia, I, of the object rotating, in this case the bar
and cylindrical weights. They are related by:
𝐼
𝐷
𝑇 = 2𝜋√
The value of D is written on each torsion axle. Note it here.
58
To investigate the influence of mass distribution vary the position of the cylinders along
the torsion bar, measure the period of oscillation, T, and use the equation above to
calculate the moment of interia, I for the combined system of cylinders plus rod. (To
improve the precision with which you measure T take the average over 10 oscillations.)
r – Position of
the cylinders
along rod (m)
T – Period of
oscillation (s)
I for the combined
system of cylinder +
bar (kgm2)
I for the cylinders
(kgm2)
0.05
0.10
0.15
0.20
0.25
Just as you can simply add two masses together to get the total mass (e.g. if the mass
of a cylinder is 0.24kg and the mass of a bar is 0.2kg, then the mass of cylinder plus bar
is 0.44kg) you can also add moments of inertia together. Given that the moment of
inertia of the bar is 0.00414 kgm2, will let you fill in the fourth column in the table above.
Plot the value of Icylinder against the position r.
Plot the value of Icylinder against the position r2.
What do you conclude from the data above and from the dependence of I from the 2
plots?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
59
Use JagFit to create a graph of Icylinder against the position r and attach your printed
graph below.
60
Use JagFit to create a graph of Icylinder against the position r2.and attach your printed
graph below.
61
The last measurement in the table above, with r= 0.25m, returns the bar + cylinders
system to the orientation used in your first investigation. You have thus measured the
moment of inertia of the bar + cylinder system in two independent ways. Write down
the two answers that you got.
I from investigation 1:
I from investigation 2:
How do the two values compare?
answers.
Which is more precise? Give reasons for your
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
62
Graphing
Many of the experiments in the 1st Year laboratory involve the plotting of a
graph. Graphs are very important in Physics as they provide a simple display of the
results obtained in an experiment and of the relationship between two variables. More
accurate and reliable information can be obtained from a graph than from the analysis
of any particular set of results.
Plotting graphs by hand:
(1)
Scale: It is important to choose the scales so as to make full use of the squared
page. The scale divisions should be chosen for convenience; that is, one unit is
either 1, 2 or 5 times a power of ten e.g. 0.5, 5, 100 etc., but never 3, 7, 9 etc.
(2)
Marking the points: Readings should be indicated on the graph by a ringed dot 
and drawn with pencil, so that it is possible to erase and correct any unsatisfactory
data.
(3)
Joining the points: In the case of a straight line which indicates a direct proportion
between the variables, the ruler is positioned so that the line drawn will pass
through as many points as possible. Those points which do not lie on the line
should be equally distributed on both sides of the line. A point which lies away
from this line can be regarded as ‘doubtful’ and a recheck made on the readings.
In the case of a curve, the individual experimental points are not joined with
straight lines but a smooth curve is drawn through them so that as many as
possible lie on the curve.
(4)
Units: The graph is drawn on squared page. Each graph should carry title at the
top e.g. Time squared vs. Length. The axes should be labelled with the name and
units of the quantities involved.
(5)
In the case of a straight-line graph, the equation of the line representing the
relationship between the quantities x and y may be expressed in the form
y = mx + c
where m is the slope of the line and c the intercept on the y-axis. The slope may
be positive or negative. Many experiments require an accurate reading of the
slope of a line.
63
Using JagFit
In the examples above we have somewhat causally referred to the ‘best fit’ through the
data. What we mean by this, is the theoretical curve which comes closest to the data
points having due regard for the experimental uncertainties.
This is more or less what you tried to do by eye, but how could you tell that you indeed
did have the best fit and what method did you use to work out statistical uncertainties on
the slope and intercept?
The theoretical curve which comes closest to the data points having due regard for the
experimental uncertainties can be defined more rigorously3 and the mathematical
definition in the footnote allows you to calculate explicitly what the best fit would be for a
given data set and theoretical model. However, the mathematics is tricky and tedious,
as is drawing plots by hand and for that reason....
We can use a computer to speed up the plotting of experimental data and to improve
the precision of parameter estimation.
In the laboratories a plotting programme called Jagfit is
installed on the computers. Jagfit is freely available for
download from this address:
http://www.southalabama.edu/physics/software/software.htm
Double-click on the JagFit icon to start the program. The working of JagFit is fairly
intuitive. Enter your data in the columns on the left.



Under Graph, select the columns to graph, and the name for the axes.
Under Error Method, you can include uncertainties on the points.
Under Tools, you can fit the data using a function as defined under
Fitting_Function. Normally you will just perform a linear fit.
One of the experiments in this manual was developed by the Physics Education Group,
CASTeL, Dublin City University.
If you want to know more about this equation, why it works, or how to solve it, ask your
demonstrator or read about ‘least square fitting’ in a text book on data analysis or statistics.
64
3