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102M Lab Manual - Department of Physics - The University of Texas

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102M Lab Manual
University of Texas at Austin
Last revised: August 19, 2014
Copyright вћћ 2014 by
The University of Texas at Austin,
Department of Physics
All rights reserved.
Printed in the United States of America
PHY102M
Laboratory Manual
Laboratory for PHY302K
Fall 2014 Edition
http://www.ph.utexas.edu/~phy102m
Department of Physics
University of Texas at Austin
Contents
Acknowledgments
v
Preface
vii
About the Lab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Understanding Basic Concepts of Physics . . . . . . . . . . . . . . . . . . . . . . . . . vii
Survival Guide
Icon Guide . . . . . .
Equation Guide . . . .
Answer Guide . . . . .
Performance Problems
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viii
. viii
. viii
. viii
. viii
0 Introduction to Experimental Physics
0.1 Experimental Physics . . . . . . . . .
0.2 Lab Methods . . . . . . . . . . . . . .
0.3 Lab Tools . . . . . . . . . . . . . . . .
0.4 Lab Report . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . . . . .
Prelab . . . . . . . . . . . . . . . . . . . . .
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1
. 1
. 1
. 6
. 7
. 11
. 15
1 Kinematics of Free Fall
1.1 Introduction . . . . . .
1.2 Background Discussion
1.3 The Free Fall Ball . .
1.4 Preparation . . . . . .
Guidelines . . . . . . . . . .
Worksheet . . . . . . . . . .
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2 Newton’s Laws and Vector Addition
2.1 Introduction . . . . . . . . . . . . . .
2.2 Vectors in 2D . . . . . . . . . . . . .
2.3 Newton’s Laws . . . . . . . . . . . .
2.4 The Force Table . . . . . . . . . . .
Guidelines . . . . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . . . .
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29
29
29
31
31
34
35
3 Newton’s Second law for Translation
3.1 Introduction to Motion . . . . . . . .
3.2 The Atwood Machine . . . . . . . .
3.3 Deriving the Acceleration . . . . . .
3.4 The Experimental Setup . . . . . . .
Guidelines . . . . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . . . .
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39
39
39
40
41
44
45
i
ii
CONTENTS
4 Conservation of Energy
4.1 Introduction to Energy . . . . . . . .
4.2 Back to the Atwood Machine . . . .
4.3 Corrections for the Atwood Machine
4.4 The Experimental Setup . . . . . . .
Guidelines . . . . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . . . .
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47
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49
50
52
53
5 Momentum Conservation in Collisions
5.1 Introduction to Momentum . . . . . . . . . .
5.2 Collisions . . . . . . . . . . . . . . . . . . . .
5.3 Special Case: One Object Initially Stationary
5.4 Carts on a Track . . . . . . . . . . . . . . . .
Guidelines . . . . . . . . . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . . . . . . . . .
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55
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55
57
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62
63
6 Rigid Body Equilibrium
6.1 Introduction . . . . . . . .
6.2 Torque . . . . . . . . . . .
6.3 Conditions for Rigid Body
6.4 The Torque Contraption .
Guidelines . . . . . . . . . . . .
Worksheet . . . . . . . . . . . .
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65
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. . . . . . .
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Equilibrium
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7 Newton’s Second Law for Rotation
7.1 Introduction . . . . . . . . . . . . . .
7.2 Derivation . . . . . . . . . . . . . . .
7.3 Moment of Inertia for a Solid . . . .
7.4 Conservation of Angular Momentum
7.5 The Rotating Disk: Inertia . . . . .
7.6 Angular Momentum Conservation .
Guidelines . . . . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . . . .
8 Archimedes’ Principle
8.1 Introduction to Archimedes’
8.2 Density . . . . . . . . . . .
8.3 Buoyant force . . . . . . . .
8.4 Suspended mass . . . . . .
8.5 Geometric Volume . . . . .
8.6 Preparation . . . . . . . . .
Guidelines . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . .
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Principle
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9 Simple Harmonic Motion
9.1 Introduction to Harmonic Motion
9.2 Springs . . . . . . . . . . . . . .
9.3 Pendulums . . . . . . . . . . . .
9.4 Full Correction . . . . . . . . . .
Guidelines . . . . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . . . . .
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83
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91
91
93
94
95
98
99
CONTENTS
iii
10 Standing Waves on a String
10.1 Introduction to Waves . . .
10.2 Standing Waves . . . . . . .
10.3 Waves on a String . . . . .
10.4 Waves on a Plate . . . . . .
10.5 Preparation . . . . . . . . .
Guidelines . . . . . . . . . . . . .
Worksheet . . . . . . . . . . . . .
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101
101
101
102
104
104
108
109
11 Heat Energy
11.1 Introduction to Heat . . .
11.2 Heat is Energy . . . . . .
11.3 The Setup . . . . . . . . .
11.4 Preparation and Warnings
Guidelines . . . . . . . . . . . .
Worksheet . . . . . . . . . . . .
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111
111
111
112
115
118
119
Appendix A Spreadsheet Basics
A.1 Layout . . . . . . . . . . . . .
A.2 Features . . . . . . . . . . . .
A.3 Graphing . . . . . . . . . . .
A.4 Printing . . . . . . . . . . . .
A.5 Further Help . . . . . . . . .
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121
121
122
123
124
124
Appendix B Units and Quantities
B.1 Common Quantities with Units
B.2 Unitless Quantities . . . . . . .
B.3 Measured Quantities . . . . . .
B.4 Common Moments of Inertia .
B.5 Greek Letters . . . . . . . . . .
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125
125
125
126
126
127
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Appendix C Capstone Lab Software
128
C.1 Capstone Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Appendix D Useful Equations
129
Index
133
Bibliography
134
iv
CONTENTS
Acknowledgments
The first edition of this manual was written by P. R. Antoniewicz in 1984 and extensively revised by David
Y. Chao in 1990. During the 1993-94 academic year a new, computer-based version of the laboratory
was developed by J. David Gavenda and Michael D. Foegelle.
Helpful suggestions for improving the experiments and clarifying the manual were made during
the 1995-96 academic year by Teaching Assistants James Daniel, Daniel Goldman, Nathan Harshman,
Robert Luter, and Paul Patuleanu.
The computer-based version of this laboratory course, which was introduced during the 1993-1994
academic year, was largely based on an IBM package called Personal Science Laboratory (PSL), consisting of probes and software. The original IBM software was revised and rewritten by Foegelle and
Gavenda specifically for 102M.
In 1997, Team Labs. Inc. introduced new Excel based software called Excelerator to run the original
PSL probes. Beginning in Spring 1998, this new software and some new probes were introduced into
the 102M laboratory course. The manual was extensively revised by Linda Reichl, Robert Luter, and
Nathan Harshman to accommodate this new software. Some of the experiments have been revised and
the procedures the students must follow have been streamlined.
In spring 2001 an updated version of the software called Excelerator 2001 was introduced in the
lab. The hardware interface was also changed from the IBM PSL to the Thinkstation interface made
by Team Labs. Where necessary, the laboratory procedures were revised in the 2002 edition by Anil
Shaji to be compatible with the new hardware and software with minimal changes to the experiments
themselves.
In spring 2004 the experiments were extensively re-designed to use the PASCO SCIENCEWORKSHOP interface and DATASTUDIO software. The new experiments and laboratory procedures were
designed by Anil Shaji. This edition of the lab manual is designed to be used with the SCIENCEWORKSHOP interface, DATASTUDIO software and PASCO probes and sensors. Some illustrations
from the previous editions have been used in the 2004 edition.
In the Spring and Summer of 2006 further changes were made to the lab manual mainly by Michael
Snyder to accommodate some hardware changes made to labs 3-5 as well as some grammatical corrections.
Lab 10 was changed by Nathan Erickson to accommodate new hardware as well. Appendix B was added
by Zhen Wei to help explain the Vernier Calipers. In the Spring and Summer of 2007 further changes
were made to the lab manual. Most of the changes were corrections for consistence and clarification of
the lab. Appendix C was also added to give the student a reference for units which are used in this lab.
These changes were found and corrected by Melissa Jerkins, Megan Creasey, Guru Khalso, Zhen Wei
and Nathan Erickson.
In the 2008 edition, Lab 1 was completely changed. Lab 11 was also changed to eliminate mm and
kg. Finally, every mention of “slope (y,x)” has been changed to “slope (y vs. x)” to try to eliminate the
confusion with variables. Thanks again to the TAs of the 2007-2008 School year for their suggestions
and corrections as well as the classes that were the guinea pigs of the new version of Lab 1.
In the 2009 editions, most changes were grammar, clarification, or consistency changes. The useful
equations section in Lab 5 was changed to use “pushed (push)” and “stationary (stay)” notation. There
were also changes to Lab 11 to work out some of the issues. A place for the lab partner’s names was
added to all the labs. Thanks again to the TA’s of 2008-2009 who were responsible for finding all the
corrections.
In the 2010 editions, most of the changes were again grammar, clarification and consistency. The one
major change was with Lab 7, where some of the current experiment was reduced and conservation of
angular momentum was added. Thanks to all the TA’s for helping with the changes and to the students
in the first section of the week who again were guinea pigs for some lab changes.
In the 2011 editions, most of the changes were again grammar, clarification and consistency. Table
number blanks were added at the top of the lab pages to assist the TA’s. The Excel addendum was
updated by Henry Schreiner to reflect the 2010 version of Excel used in the computer labs. Thanks to
all the TA’s and students who helped find errors in the lab manual.
v
vi
ACKNOWLEDGMENTS
In the 2013 edition, lab 0 was rewritten in preperation of a new version of the manual.
In the 2014 edition, the manual was heavily revised and reformatted, and several labs were updated.
New equipment was incorporated into the labs. Lab 1 now uses calipers and rulers. Lab 2 was reworked
to enhance comprehension. Lab 5 was rewritten for all new equipment, and restructured for easier data
entry. Lab 11 was reworked. Food for thought and notation corners were added, and an index was
added. Places for taking notes were added to all labs. A Preface and a Survival Guide were added. All
new figures and schematic diagrams were added throughout. New software are hardware required many
changes to procedures. New slope fitting was added to several labs. Clear all data runs was removed
from the labs and the manual. The Excel addendum was changed to be a general spreadsheet tutorial,
with individual tutorials for specific programs moved to the website.
Preface
About the Lab
This course, PHY102M, provides an opportunity to test, in a laboratory setting, some of the
basic laws that govern the physical world, and it will help you develop an intuitive understanding
of these basic laws. At the same time, you will learn about standard laboratory reporting
procedures and the role of experimentation in testing the basic laws of physics.
PHY102M is designed to be taken concurrently with PHY302K, which provides the theoretical basis for the experimental work done in PHY102M. However, the course and the lab are
separately run and taught, and the lab is designed to be self-contained. Topics in the lab may
be covered either before or after they are covered in PHY302K.
Understanding Basic Concepts of Physics
Physics deals with everyday phenomena, but they must be expressed in terms of certain basic
concepts before the laws of physics can be applied. For example, Newton’s Second Law of
Motion accurately predicts the motion of an object, but it does so by giving the acceleration of
the object when forces act on it. You cannot hope to use the Second Law until you understand
what “acceleration” and “force” mean, and you soon find out that simply memorizing their
definitions is not very helpful.
In this course you will learn, for example, to find the acceleration of an object from measurements of its position at various times. You will learn how to measure the forces acting on
objects in order to apply the Second Law. In the process of carrying out the experiments you
should begin to make the mental connections between the definitions of the concepts and your
real-world experiments.
vii
Survival Guide
Icon Guide
When completing the labs, look for icons to indicate extra pages required. The icon indicates
that you need to complete this on a computer and print it (in a computer lab or at home), the
icon indicates a page printed during the lab, and the
icon indicates a handwritten page.
These are the lines used to mark pages needed:
Include graph: y vs. x в†’ Fit equation:
в†’ This
tells you to include a graph printed at home, and the order of the variables tells you which one
is on each axis.
Include printout: Name of printout в†’ This tells you to include a printed page from lab.
Include page: Type of page в†’ This is for handwritten pages (usually worked examples).
Include postlab в†’ This reminds you about including a postlab printed at home (every lab).
Equation Guide
Displayed equations come in three forms. Numbered equations are important results and often
are useful for the final exam. Numbered equations with a star are useful for doing the lab (high
probability that you will use this exactly as it appears). Unnumbered equations are there just
to explain how other equations are derived.
Answer Guide
There are several kinds of answer blanks.
1. The normal answer blank,
, is usually for a single number and a unit.
2. The uncertanty answer blank,
В±
, expects an uncertainty also.
3. The name and number answer blank,
=
, expects you to write down
the name used in the manual (a variable, like x or r), and the number with units.
Performance Problems
Most common causes for poor performance in the lab are:
вќј Not reading the lab beforehand.
вќј Not watching the prelab videos.
вќј Forgetting to fill in an answer.
вќј Forgetting units.
вќј Not looking in the appendix for help with Excel or with units.
вќј Not checking your calculations if you have a large percent error. (If error persists, make
a note saying you checked it.)
All TA’s have office hours to help if you don’t understand a question or a graded mistake. If
you have trouble, check the UT 102M website: www.ph.utexas.edu/~phy102m. This has links
to further help on the labs, such as interactive applets. This also has links to the prelab videos.
viii
Introduction to Experimental Physics
Lab 0
0.1
Experimental Physics
0.1.1 Scientific Proof
Most people know that scientific hypotheses or theories are tested by performing appropriate
experiments. It is less well known, however, that experiments can never prove, in the mathematical sense, that a theory might happen to give the same results for a given set of experimental
conditions. At most, experiments can be used to show that a theory is probably false. This
would be the case if the experimental results contradicted the predictions of the theory. As a
consequence, scientists spend much of their time trying to show, by a wide variety of experiments, that some theory is false. If no one succeeds after many attempts, the theory comes to
be accepted as a law.
0.1.2 Properties of Matter
Experiments are not only used to test scientific hypothesis, they can also be used to measure
basic properties of material objects, such mass density, heat capacity, force constants, etc. We
will do some of that in the laboratory course.
0.1.3 Experimental Conclusions
The result of any experiment will be one or more numerical values, expressed with an estimate
of their uncertainty and units. The final step is to draw a conclusion about the hypothesis or
theory being tested on the basis of these values: Are the results consistent with the predictions
of the theory to within the uncertainty inherent in the experiment? If not, why not? Is the
theory wrong, or is it possible that systematic errors caused the discrepancy? Any statements
about the effects of a systematic error must be logically consistent. For example, if you say
that friction in the apparatus caused the measured acceleration to differ from that predicted
by the theory, you must show that friction would lead to a lower acceleration if your measured
value is lower than that predicted, and vice versa.
0.2
Lab Methods
0.2.1 Measurement Uncertainty
All measurements have some degree of uncertainty built in simply from the limitations of the
measuring devices. Therefore, every measurement that we make in the lab must be reported
with the uncertainty due to the measuring device. Below we give “rules” for specifying the
error intrinsic to various types of measurement.
Ruler Rule
We will use the rule that a measurement has an uncertainty equal to 1/2 the smallest
interval on the measurement device. You must guess a digit in the smallest interval to
use this rule. This is used, for example, with rulers, meter sticks, and Vernier calipers if you
are good at them. For example, if a length is measured with a meter stick, the smallest interval
1
2
LAB 0. INTRODUCTION TO EXPERIMENTAL PHYSICS
marks are 1 mm apart. Therefore, the uncertainty of a length measurement for this measuring
device is 0.5 mm or ОґL = В±0.0005 m. You must guess an extra digit when you read the meter
stick, so the reading might look something like 7.3 В± 0.5 mm. You should never report this as
7 В± 0.5 mm, as the decimal place of uncertainty and the reading would not match.
Digital Rule
Many devices (including some we will use in this lab) give a digital readout of the data they
are measuring. For such devices, the uncertainty is equal to the smallest interval in the
digital readout.
Fluctuating Rule
If the reading fluctuates (some digital readings), the uncertainty is equal to the size of
the fluctuations. This may occur in two situations. First, if a device can only be read in
increments of 0.03 N, for example, it is common in scientific applications to have the device
report all available digits—but this has an uncertainty of about 0.03 N, not 0.01 N, since it
can’t read any numbers in between its minimum increment. (Many commercial applications,
like phones, will actually display fewer digits to hide this resolution limit).
The second reason you may see fluctuations is because something is actually fluctuating.
You are still unsure of the exact reading because it is changing.
Notation Corner
Uncertainty is denoted with the small Greek letter delta (Оґ) prefixed before the
quantity. If it is part of a measured value, it is listed with a plus/minus symbol
(В±), and comes after the number but before the unit. If you have best measurement x0 , and x0 = 2.20 m and Оґx = 0.01 m, the correct way to write it is in the form
x = x0 В± Оґx, with the unit last:
x = 2.20 В± 0.01 m
0.2.2 Significant Figures
The number of reliably known digits in a number is called the number of significant figures.
When making measurements, or when doing calculations, you should not keep more digits in
the final answer than is justified. As a general rule, when adding or multiplying numbers, the
final result should have only as many significant figures as the least accurate number used in
the calculation. When performing a calculation, always wait until you are writing your final
answer and then write the final answer with the correct number of significant figures.
The number of significant figures in a stated number may not always be clear. For example, if someone tells you the distance between two cities is about 920 miles, does this mean
920 В± 10 miles or 920 В± 1 miles? However, when using scientific notation (numbers expressed in
exponential notation), it can always be made clear—the number of decimal places kept is the
number of significant figures. For example, the number 920 В± 10 is written 9.2 Г— 102 , indicating that it has two significant figures. The number 920 В± 1 is written 9.20 Г— 102 , indicating
that it has three significant figures. The number of decimal places kept tells us the number of
significant figures.
Addition/subtraction matches the largest final decimal place, multiplication/division keeps
the smallest number of significant figures.
0.2. LAB METHODS
3
Significant Figures – Worked Example
Consider a number w = x + yz which is composed two numbers y = 1.0 and z = 1.00
which are multiplied together and then added to a third number x = 9.000. Write
the number w with the correct number of significant figures (sig figs).
....................................................................................
Note that y = 1.0 has 2 sig figs and z = 1.00 has 3 sig figs. The product has 2 sig
figs, so yz = 1.0.a Now add yz to x to get w = 9.000 + 1.0 = 10.0, so w has three sig
figs, since when adding we match the last decimal place.
To demonstrate this, we can replace unknown digits with question marks ?. Unknown digits (question marks) stay unknown (since they can be any number from 0
to 9) when they are added or multiplied. So, for example,
1.0??
+9.000
1.0??
a
Don’t remove digits from your calculator! Keep extra digits until you are done if you can.
0.2.3 Uncertainty in Calculated Quantities
Every calculation we make using measured quantities will have an uncertainty associated with
it. Let us assume that two quantities, A and B are measured, with measurement uncertainties of
ОґA and ОґB, respectively. These measured quantities are then written in the form A = A0 В± ОґA
and B = B0 В± ОґB, where A0 and B0 are the best measured values of the quantities A and
B. Below we show how to add or subtract, multiply or divide, these numbers and obtain the
uncertainty ОґC of the resulting calculated number C = C0 В± ОґC.
Addition and subtraction
If we calculate the sum, C = A+B, we can replace these with the full expressions for uncertainty,
C = C0 В± ОґC = A0 + B0 В± ОґA В± ОґB
Thus, the calculated quantity has value C0 = A0 + B0 and its uncertainty is ОґC = ОґA + ОґB.
Note that one must be careful when subtracting two numbers. Never subtract the errors.
Always add them. Subtraction would result in C = A в€’ B = A0 в€’ B0 В± ОґA В± ОґB, and therefore
the same final formula for error.
ОґC = ОґA + ОґB
(0.2.1)
Multiplication and division
If we multiply numbers, C = AB, we can again expand
C0 В± ОґC = (A0 В± ОґA)(B0 В± ОґB)
If this product is multiplied out, we get
ОґC = A0 ОґB + B0 ОґA + ОґAОґB
The final term, ОґAОґB, is very tiny as long as the uncertainties are smaller than the best values,
so we can safely remove it.
4
LAB 0. INTRODUCTION TO EXPERIMENTAL PHYSICS
ОґC = A0 ОґB + B0 ОґA
(0.2.2)
We can use the above formula if needed for multiplication problems, or we can go a few steps
further to get a more general form we can use with division too. We divide both sides by A0 B0
to get
ОґA ОґB
ОґC
=
+
A0 B 0
A0
B0
Remembering that A0 B0 = C0 finally gives us the formula for error:
ОґC
ОґA ОґB
=
+
C0
A0
B0
This is sometimes called relative error, because each term is the fractional error of the
quantities. You can work this through for division1 This multiplication and division rule is
most commonly written in a more useful (but possibly harder to remember) form:
ОґC = C0
ОґA ОґB
+
A0
B0
(0.2.3)
Raising to a power
Most common operations are now accounted for, but let’s include a quick short-cut for raising
to a power (C = An ). We can just take the multiplication rule over and over again, and the
result is
ОґA
ОґC = nC0
(0.2.4)
A0
Measurement Uncertainty – Worked Example
Let us consider the number W = X + Y Z which is composed of three measured
numbers X = 9.000 В± 0.001, Y = 1.0 В± 0.1 and Z = 1.00 В± 0.01. Compute W
including its uncertainty.
....................................................................................
Using the notation of Section (0.2.3), X0 = 9.000, ОґX = 0.001, Y0 = 1.0, ОґY = 0.1,
Z0 = 1.00, and ОґZ = 0.01. First multiply Y and Z to get Y Z = Y0 Z0 В± (Y0 ОґZ +
Z0 ОґY ) = 1.0 В± 0.1. Now add this to X to get W = 10.0 В± 0.1.
0.2.4 Error and Percentage Error
There may be error sources, both systematic and random, not accounted for by the measurement
uncertainty of our measuring devices or by our theory. For example, in experiments which test
the laws of dynamics, we try to eliminate the effects of friction and of air resistance as much as
possible, but they can never be entirely eliminated. Both of these effects will cause systematic
deviations of our results from our theoretical predictions.
1
Hint: Any time you have two Оґ quantities multiplied by each other, the result is very small
0.2. LAB METHODS
5
0.2.5 Absolute Percentage Error
When performing a measurement of some physical quantity, we need to find as many ways as
possible to check to see our results are reasonable and can be trusted. Have sources of error
crept in that we are unaware of? If the experiment measures some quantity, V , for which there
is a known “accepted” value, Vaccept , coming from other careful experiments, then we might
calculate the percentage error, EV (%), of our result, Vmeas , from the “accepted” value, Vaccept .
The percentage error is defined as
EV (%) =
Vaccept в€’ Vmeas
Г— 100%
Vaccept
(0.2.5)
The percent error gives an estimate of the discrepancy between our measured result and
the accepted result. If the discrepancy is larger than expected considering the known inherent
errors in the experiment, then we must re-examine all aspects of the experimental procedure
until we understand the source of the large discrepancy.
This also is a good way to test error in a conserved quantity; in this case, you would replace
accepted with initial, and measured with final in this formula.
0.2.6 Relative Percentage Error
Sometimes we do a measurement of some quantity, V , for which there is no known accepted
value. However, if we measure V in two different ways and get results, V1 and V2 , then we can
check the internal consistency of our measurement process by computing a relative percentage
error
V1 в€’ V2
EV (%) =
Г— 100%
(0.2.6)
(V1 + V2 ) /2
We will use both the absolute percentage error and the relative percentage error in this
laboratory course.
0.2.7 Units of Physical Quantities
All physical quantities have “units” associated with them, and the size of these units is determined by international agreement. In this lab, we generally use the S.I. system of units,
which is the system of units most appropriate for scientific purposes. In the S.I. system, length
is measured in meters (m), time in seconds (s), mass in kilograms (kg), and temperature in
Kelvin (K). We will encounter a number of physical quantities whose units are combinations of
the basic units mentioned above. For example, energy is measured in units of Joules (J) and
1 J = 1 kgВ·m2/s2 .
Whenever a physical quantity is measured, the reported value must include both the numerical value and the unit associated with that physical quantity. For example, if an experiment
measures the acceleration of gravity, the reported value would be of the form g = 9.8 В± 0.1 m/s2
(if the error in the measurement were of order 0.1 m/s2 ). If your numbers do not include
units, you will lose points!
Three important facts about units:
1. Units can be multiplied and divided according to the laws of algebra.
2. Units that are not equivalent (mass and length, for example) cannot be added or subtracted.
6
LAB 0. INTRODUCTION TO EXPERIMENTAL PHYSICS
3. For every equation that you write down, the units of each term in the equation are the
same.
This provides an important way to check if your equation is correct. For example, the
displacement x at time t of a particle that experiences constant acceleration a can be written
in terms of the displacement x0 at time t = 0 s, the velocity v0 at time t = 0 s as
1
x = x0 + v0 t + at2
2
with units в†’ m + (m/s) s + (m/s2 ) s2 = m
Notice that after cancellations (according to the rules of algebra), every term in this equation
has units of meters.
0.3
Lab Tools
0.3.1 Vernier Caliper
0
0
5
5
10
10
15
20
25
30 mm
0.1 mm
Figure 0.1: A closed Vernier Caliper.
A Vernier caliper is a measuring tool designed to get better readings than a ruler. A caliper
is just a device with a sliding jaw that you put on something to measure it. Usually there are
outer jaws to measure objects, and inner jaws to measure cavities. Most calipers also have a
Vernier scale. This is a special scale that uses a simple mathematical trick to enable better
readings.
A Vernier scale is designed by matching two separate scales at slightly different spacing.
For the pictured caliper, the lower (Vernier) scale has 10 marks in 9 mm. This means the first
mark is at 0, followed by 0.9 mm, then 1.8 mm, etc. If you ignore the whole number part and
only look at the decimal, you’ll see that it decreases by 0.1 mm each time. This is the trick that
is used; for it is easier for you to see which set of lines matches than to see lines 0.1 mm apart.
To read a caliper, first read the main scale as you would a normal scale. The point you
need to read is the location of the 0 vernier line on the normal scale (look at a closed caliper,
the reading should be 0, see Figure 0.1). Then, to get another digit, read the vernier scale by
matching lines. If the 3rd line past 0 matches a line on the main scale, the vernier reading is 3.
(See Figure 0.2)
0.4. LAB REPORT
7
6.3 mm
First, read 6 mm from main scale
0
5
10
0
15
5
10
20
30 mm
25
0.1 mm
Then get final digit, .3 mm
by matching the third line
6.3 mm
Figure 0.2: An open Vernier Caliper.
Always make sure your reading is reasonable. If you would have guessed the measurement
to be between 6 mm and 6.5 mm, your vernier reading should come out to be between 6 mm
and 6.5 mm.
Even though your first time or two reading a vernier scale may be challenging, with practice,
you will find they are fast and easy to read.
0.4
Lab Report
At the end of each chapter there is a Lab Report section which will help guide you in taking
data during the experiment, and in analyzing that data afterwards. This set of sheets can
be torn out of the lab manual and must be turned in as part of your lab report. Your Lab
Instructor will give you additional questions and topics to discuss in a post-lab that also part
of your lab report.
0.4.1 Graphs
x vs. t
t (s)
0.3
1.3
3.5
5.5
x (m)
1.1
1.5
2.0
2.9
x (m)
3
x = 0.333t + 1
2
1
1
2
3
t (s)
4
5
6
Figure 0.3: An example graph.
A graph provides a visual picture of your data, and can show qualitative features not obvious
from looking at the numbers alone. Figure 0.3 shows a set of data along with it’s matching
graph, and a linear fit line.
8
LAB 0. INTRODUCTION TO EXPERIMENTAL PHYSICS
Often your labs will require you to make graphs in a program such as Excel. These should
be done at home, or on a flash drive or personal computer in lab. You should make your own
graph, you cannot share anything except raw data with your partners. A graph should always
include the following parts:
Title The “vs.” always indicates vertical axis versus horizontal axis.
Labels on both axes Include units.
Tick marks The tick marks provide numerical increments for the values of plotted quantities
(grid optional).
Points Always use distinct points, never use a connect-the-dots form.
Fit line We shall often be asked to plot a curve that best fits the data obtained in an experiment. This can be obtained either by hand or by using a software package like Excel.
Fit equation It is often possible to write an equation that represents the curve that best fits
your data. This is particularly simple if the curve is straight line. If the curve is not a
straight line then a software package like Excel can be used to find the fit. Excel uses y
and x to refer to the vertical axis and the horizontal axis, respectively.
Your graphs should be as large as possible, usually a full page. Make sure you know what
fit is needed (linear or polynomial order 2 in this lab). It is possible to have points that look
like a line that are described by a second order polynomial equation. You will often compare
the equation of a graph with an equation from lab to find the values of the coefficients.
For a graph (with vertical axis y and horizontal axis x) whose data is fit by a straight line,
the equation describing that empirical curve can be written in the form y = mx + b, where m
is the slope of the best fit curve and b is the y-intercept. You can obtain the average slope m
between two points y2 = y(x2 ) and y1 = y(x1 ) by writing
m=
y2 в€’ y1
∆y
,
=
x2 в€’ x1
∆x
where ∆y = y2 − y1 and ∆x = x2 − x1 .
For example, given the data and straight-line curve in Figure 0.3, we can write x = v0 t + x0 ,
where the slope of the curve is v0 = 0.333 m/s and the intercept is x0 = 1 m.
Lab 0
Notes
This is for initial comments, work, etc. that you may have from the reading and the prelab
videos. Put down anything you think might help for the upcoming lab.
9
10
LAB 0. INTRODUCTION TO EXPERIMENTAL PHYSICS
Lab 0
Worksheet
Name:
Table:
Partner(s):
Note: This lab worksheet will not be graded. This is intended to prepare you for working in
this lab.
Significant figures
What is the correct way to report d = 0.13724 В± 0.00247m?
Given the following numbers, write the answer with the correct number of significant figures.
Include units.
A = 3.000 m, B = 1.11 m, C = 0.004 m, D = 2.02 s
AГ—B =
AГ—C =
A/B =
A/D =
A5 =
Error Propogation
Given the following numbers, write the answer with the correct error and signfificant figures.
Include units.
A = 3.000 В± .005 m,
B = 1.11 В± .05 m,
C = 0.004 В± 0.001 m,
D = 2.02 В± .08 s
AГ—B =
AГ—C =
A/B =
A/D =
A5 =
Assume a = 12 В± 2, b = 3.0 В± .5, c = 2.0 В± 0.2, and d =
uncertanty) of d:
11
a
b
в€’ c. Then, compute the value (without
12
Lab 0. Worksheet
d0 =
Now, compute the maximum value that d can be (hint: a should be a = 12 + 2 = 14. Expermiment to maximize d).
dmax =
Do the same to find the minimum value for d.
dmin =
Now, propogate the uncertanty as seen in section 0.
Оґd =
Compare dmax with d0 + Оґd, and dmin with d0 в€’ Оґd.
Lab 0. Worksheet
13
Graphing
Fill in the missing parts of this graph of d (distance in meters) vs. t (time in seconds).
5
y = 0.500x + 1
4
3
2
1
1
2
3
4
5
6
14
Lab 0. Worksheet
Verneir Calipers
Read the following calipers (include uncertanty and units, as always):
0
5
0
10
5
15
10
20
25
30 mm
20
25
30 mm
0.1 mm
Reading =
Read the following calipers:
0
5
10
0
15
5
10
0.1 mm
Reading =
Measure the length of the object the cylinder in your box. Record that here:
Length =
Include postlab
TA sign-off:
Lab 1
Lab 1 Prelab
Name:
x (m)
x vs. t
9
8
7
6
5
4
3
2
1
0
0
0.5
1
1.5
t (s)
2
2.5
3
1. A ball is rolling along the x-axis as shown above, under constant acceleration. Solve the
following. Show your work.
(a) Find its displacement, ∆x1 , during the time from 0 to 1 seconds.
(b) Find its average velocity, vВЇ1 , during the time from 0 to 1 seconds.
(c) Find its average velocity, vВЇ2 , during the time from 1 to 2 seconds.
(d) What is t when the object’s velocity, v1 , is equal to its average velocity, v¯1 , during
the first time interval? Think about the shape of the v-t graph.
15
16
Lab 1. Prelab
2. Read the following Vernier caliper:
0
5
10
0
15
5
10
20
25
30 mm
0.1 mm
Reading =
3. Calculate and insert in the tables below the values missing from the tables in the description of the experiment.
j
xj (cm)
0
2.5
1
8.2
2
13.0
3
16.4
4
19.0
Tj is the time at which the j th video frame is captured. The position, xj , is the position
of the steel ball in the j th frame. (The data is taken 30 times per second.)
j
tj (s)
xj (m)
0
0.0
0.0250
1
0.0333
0.0820
2
0.0667
0.130
3
4
Average Acceleration: a
ВЇtable =
j
vВЇj (m/s)
tj (s)
0.5
1.71
0.0166
1.5
1.44
0.0500
2.5
1.02
0.0833
3.5
j
a
ВЇ (m/s2 )
1
-8.10
2
-12.6
3
Lab 1
1.1
Kinematics of Free Fall
Introduction
Kinematics is the description of the motion of material objects. The description of a moving
object is a record of the position of the object as a function of time. Once this record of position
versus time is obtained, it can be used to obtain the velocity of the object (the rate of change
of position) and the acceleration of the object (the rate of change of velocity).
In this lab, we study the kinematics of an object that is undergoing two particular kinds of
motion simultaneously, constant velocity in the horizontal direction and “free fall” in the vertical
direction. An object will have constant velocity in the horizontal direction if no horizontal
force acts on it. An object undergoes free fall motion when the only force on the object is the
gravitational attraction of the Earth. One goal of this experiment is to show that the horizontal
and vertical components of motion are independent of each other.
While we can come close to ideal unaccelerated motion and free fall, neither of these perfectly
occur on Earth because of air resistance. When an object moves through the air, it experiences
a “drag” force which opposes its motion. This drag force is proportional to the squared speed of
the object and the cross sectional area perpendicular to the motion. An additional goal of this
experiment is to show that the effects of air resistance can be made small if the cross sectional
area is small and speeds are low. Then the horizontal and vertical motions experience very
small deceleration and we can measure the acceleration due to gravity during free fall with a
fair degree of accuracy.
The object whose motion we will study is a steel sphere (its small size and large inertia will
help reduce the effects of air resistance). A web cam will be used to record the position of the
steel ball, at discrete times, as it moves through the air. A grid placed behind the steel ball will
enable us to see each position of the steel ball recorded by the webcam (each frame), and the
repetition rate of the webcam will enable us to obtain the time at which each frame occurred.
The major errors in this lab will be due to the resolution of the camera and our ability to
accurately find the position of the steel ball as a function of time.
Food for thought
What type of error is introduced by the resolution of the camera?
What about the drag force? (Systematic or Random)
1.2
Background Discussion
1.2.1 Constant Velocity
For an object with a constant velocity vx along the x-direction, if the initial position is xi at
the initial time ti , the position at a later time tf is
xf = xi + vx ∆t
17
(1.2.1)
18
LAB 1. KINEMATICS OF FREE FALL
where ∆t = tf − ti . The quantity ∆x = xf − xi is the displacement of the object during the
time interval ∆t.
1.2.2 Constant Acceleration
Let us now consider a slightly more complicated set of conditions. Assume a small mass moves
freely along the x-direction under the influence of a constant force and, therefore, a constant
acceleration, a. This will cause the velocity to change as time passes. However, let us assume
that we know the initial conditions (the velocity, v0 and position, x0 at the initial time t0 = 0).
If we are able to record the position x at a time t, we can relate them to the initial conditions
and the acceleration as follows:
1
x = x0 + v0 t + at2
2
(1.2.2)
v = v0 + at.
(1.2.3)
and
These two equations are only valid if acceleration is constant. We can see that
equations (1.2.2) and (1.2.3) reduce to equation (1.2.1) if the acceleration is zero. Finally, if the
only force is due to gravity then we normally use the variable g to represent the acceleration due
to gravity. Since we usually consider the upward direction to be positive, and the acceleration
of gravity is directed downward, we have the common replacement a = в€’g.
1.2.3 Computing Averages
In order to complete our experiment, we’ll need to compute the average velocity and average
acceleration, from a series of data points, for the case when the acceleration is not constant. If
we know the position xi if an object at time ti and its position xf at the later time tf , then its
average velocity during this time interval is
vВЇ =
xf в€’ xi
∆x
=
∆t
tf в€’ ti
(1.2.4)
At what time during the motion between xi and xf is this average velocity actually reached?
A simple method is to take the average of the two times, tf and ti , called the midpoint time.
This has an added bonus; when the acceleration is constant, the average velocity is
the velocity at the midpoint time. If the acceleration is not constant, it is still a better
approximation than choosing one of the two endpoint times.
We can also find average acceleration (shown in Figure 1.1):
a
ВЇ=
vf в€’ vi
∆v
=
∆t
tf в€’ ti
(1.2.5)
Even if the line is not as straight as the one in Figure 1.1, this still gives you the average
acceleration between the two chosen points.
1.2.4 The j Label
In this lab, we will follow the motion of the steel sphere with a webcam that records the position
of the sphere at discrete times tj (j = 0, 1, ...). Each record of position is called a “frame”
and the rate at which the positions are recorded is called the “frame rate”. The frame-number
subscript j, will increment starting from j = 0 (which is the first usable recorded point) to
1.3. THE FREE FALL BALL
19
v vs. t
,
(t f
5
v f)
v (m/s)
4
∆v
3
)
, vi
(t i
2
∆t
1
1
2
3
4
5
t (s)
6
7
8
9
Figure 1.1: Average acceleration example graph.
the last time recorded. Midpoint times are labeled by using half values for j. To calculate the
actual time tj for each value of j, you can simply use the frame rate f (in frames per second):
tj =
j
f
(1.2.6)
With this notation, if the label for the initial point is j в€’ 0.5, and the label for the final point
is j + 0.5, so that the average velocity equation (1.2.4) becomes
vВЇ =
We can write a similar equation a
ВЇ=
xj +0.5 в€’ xjв€’0.5
∆x
=
∆t
tj +0.5 в€’ tjв€’0.5
∆v
∆t
(1.2.7)
for the average acceleration (1.2.5).
Food for thought
All of the above equations were written using the position variable x, but they are
generally rewritten with position variable y when describing motion in the vertical
direction. Will the acceleration a be different for x and y?
1.3
The Free Fall Ball
We are going to test our theory by watching an object in free fall. Free fall occurs whenever
an object does not have any forces other than gravity acting on it. Air resistance does affect
free fall, but not significantly at slower speeds for small massive objects. We hope to be able
to measure the acceleration due to gravity, and to show that we can measure and calculate
vertical displacement y and horizontal displacement x separately for the steel sphere.
We are going to film the motion of the steel sphere with a web cam (shown in Figure 1.2),
as it is bounced past a sheet of paper with grid lines. We will use the grid lines to record the
position of the sphere at discrete times tj . You have a two-sided sheet of graph paper in your
20
LAB 1. KINEMATICS OF FREE FALL
Figure 1.2: Bouncing ball and webcam setup.
workbook to use for the two runs. The sheet is attached to the “wall” behind the bouncing
sphere.
In the first run we’ll bounce the ball and only analyze the vertical component of the motion.
The bounce allows us to track frames for the sphere going down and up, versus dropping it and
only tracking downward frames. As long as nothing touches the ball between the starting time
t0 and ending time tjmax , we have free fall.
In the second run, we will arc the ball across the paper, and track both vertical and horizontal
distances.
1.4
Preparation
You’ll need to set up your camera to record the best possible image. In BTVPro, go to Video
Device Settings and choose Adjustments. Under Color, make sure Gamma is maximum (1).
Under Image, make sure the Saturation is minimum (there is no need for a color image),
and set Sharpness, Brightness, Gain, and Shutter to their maximums (we are trying to
make the shutter stay open for as little time as possible). You will need to set any “auto”
drop-down boxes to “manual” to adjust some of the sliders. Adjust the Exposure slider to get
a reasable brightness for your image. Usually, this is somewhere between 100–200. Slightly too
dark is usually clearer than slightly too bright. See Figure 1.3. Arrows point out the locations
of the important sliders.
The framerate for the camera is set to 30 frames/second.
Make sure the entire grid behind the ball is visible and reasonably flat. Twist the ring in
front of the lens to change the focus.
1.4. PREPARATION
Figure 1.3: Proper settings for the sliders in BTVPro.
21
22
LAB 1. KINEMATICS OF FREE FALL
Lab 1
Notes
This is for initial comments, work, etc. that you may have from the reading and the prelab
videos. Put down anything you think might help for the upcoming lab.
23
Lab 1
Guidelines
Raw Data
First dataset
Film the ball in free fall for at least 8 consecutive frames, more than 8 is better. You will need
to bounce the ball and then track the part after the bounce in order to get enough frames. You
cannot include a bounce for it to be in free fall! Transfer data from the screen to the graph
paper at the end of this lab. Number the points on the graph paper to keep track of direction.
Use a ruler to measure the positions and record them in the worksheet.
Looking at the ball diameter on the screen, try to measure its apparent diameter based on the
squares behind it.
Second dataset
Repeat the first experiment, only this time bounce the ball across the paper in an arc; record
both vertical and horizontal motion.
Analysis
Tables
You’ll need to calculate both v and a for midpoint times, though only on your first dataset.
Graphing the datasets
Make the graphs indicated in spreadsheet software and include them with your lab report. Be
sure your fit lines match the form of the expected equations (1.2.3) and (1.2.2). Copy the fit
equations from the graphs, and use the coefficients to answer the questions below.
24
Lab 1
Worksheet
Name:
Table:
Partner(s):
Run 1: Vertical
Note: Always include units!
Frame Rate: fframerate =
Ball Diameter: dreal =
j
tj (s)
Note: Must measure with calipers!
yj (m)
j
0
tj (s)
vj (m/s)
j
0.5
1
1
1.5
2
2
2.5
3
3
3.5
4
4
4.5
5
5
5.5
6
6
6.5
7
7
7.5
8
8
8.5
9
9
9.5
10
10
10.5
11
Average of above values: atable =
dapperent =
Is it different from the actual diameter?
Graphing the first dataset
Include graph: y vs. t
в†’
Fit equation:
Time at the highest peak on the y graph: Tmax =
Use Tmax and the fit line eqn. to get maximum height: Ymax =
From the y vs. t graph coefficients: a =
Include graph: v vs. t
в†’
v0 =
Fit equation:
From the v vs. t graph coefficients: a =
v0 =
25
aj (m/s2 )
26
Lab 1. Worksheet
Run 2: Both Directions
j
tj (s)
xj (m)
yj (m)
0
1
2
3
4
5
6
7
8
9
10
11
Graphs of the second dataset
Include graph: y vs. t
в†’
Fit equation:
Using the values of the best fit line coefficients on the y vs. t graph:
vy,0 =
Include graph: x vs. t
ay =
в†’
Fit equation:
Using the values of the best fit line coefficients on the x vs. t graph:
vx,0 =
ax =
Include page: Work
Make sure you include the following sheet of graph paper, too, with your numbered points.
Include postlab
TA sign-off:
Lab 1. Worksheet
27
28
Lab 1. Worksheet
Lab 2
2.1
Newton’s Laws and Vector Addition
Introduction
We live in a three-dimensional world and, consequently, many of the quantities we work with
have both a magnitude and a three-dimensional direction—we call these vector quantities. We
also work with quantities that do not have a direction, but only a magnitude - we call these
scalar quantities. For example, velocity is a vector quantity. To fully specify the velocity of
a moving object, we must specify the magnitude of its velocity (its speed), and the direction
of the motion. A volume is a scalar quantity; for example, a liter or a cup does not have a
direction.
Sometimes our motion is constrained to one or two dimensions. For example, If we walk
alone a straight flat road, we are moving in one dimension. There is no vertical or sideways
component to our velocity, so we can neglect those other two dimensions. If your motion has
only one dimension, it is sufficient to just use a sign to indicate direction. One direction is
positive, and the other is negative.
In this lab, you will learn how to work with vectors in two dimensions. In Lab 1, you
actually studied motion in two dimensions, but kept them separate. In this lab, we will learn
how to express two dimensional motion in vector notation.
Notation Corner
We often use one of three ways to indicate a vector.
вќј Textbooks often use bold upright font, A
вќј Handwritten equations often use an arrow, A
вќј On the blackboard, you will sometimes see double lines, A
If a vector is not marked as a vector, it indicates the magnitude of the vector (A =
|A|).
2.2
Vectors in 2D
In this lab, you will only work with two dimensional vectors (2D vectors). However, everything
we will cover is fairly simple to extend to 3D with the right mathematics. It is just harder to
measure and visualize in 3D.
A vector can be thought of as an arrow, where the length is the magnitude of the vector, and
the direction of the arrow indicates the direction of the vector. Vectors do not have a location!
The vector arrow can be moved anywhere on the graph without changing its properties as long
as one does not change its length or its direction. This should give you an idea of how to add
vectors.
As shown in Fig. 2.1, vectors can be drawn on a graph. Their direction is always measured
in terms of the angle they make with the positive x-axis in a counterclockwise direction.
29
30
LAB 2. NEWTON’S LAWS AND VECTOR ADDITION
y
3c
m
@
13
5вћ¦
135вћ¦
x
Figure 2.1: An example vector.
Graphical Addition
Two vectors, A and B, are added by joining the tail of the B arrow to the head of the A arrow.
The resultant vector C = A + B is an arrow drawn from the tail of A (at origin) to the head
of B. The length of C can be measured with a ruler and the direction of C can be measured
with a protractor. Always include a scale.
=
+
B
A
B
C
A
1 cm = 2 N
Figure 2.2: Vector addition example.
Remember that angles are always measured counterclockwise from the x-axis (0вћ¦). The
resultant vector always points from the original starting point to the last vector tip. This
should give you a common-sense notion of what vector addition does, it is the shortest way
from the beginning to the end. As the crow flies, so to speak.
Algebraic Addition
Algebraic addition is another way to add vectors. To add, simply split up all the vectors into
perpendicular (orthogonal) components (in our case, x and y), and then add the components
like normal numbers.
Cx = Ax + Bx
A+B=C
=в‡’
(2.2.1)
Cy = Ay + By
If you are given the components Ax and Ay of a vector A, you can easily find the length
A and direction Оё of the vector. Remember that Ax = A cos(Оё) and Ay = A sin(Оё). Then
A=
A2x + A2y and tan Оё =
Ay
Ax .
However, note that there can be some ambiguity regarding
A
the angle Оё. If you use Оё = tanв€’1 Axy , your calculator may only give answers from в€’90вћ¦ to
90вћ¦ (in the first and fourth quadrants), even if the correct answer lies in the second or third
quadrants (between 90вћ¦ and 270вћ¦). You can recover the correct angle (90вћ¦ to 270вћ¦) by adding
180вћ¦ to the answer if Ax < 0. If you want to write a positive angle, you can always add 360вћ¦ to
the angle without changing it.
As an example, let us consider the vector shown in Fig. 2.1, which we will call A. The
length of A is A = 3 cm and the angle it makes with the positive x-axis is Оё = 135вћ¦. Thus,
Ax = (3 cm) cos (135вћ¦) = в€’2.12 cm
and Ay = (3 cm) sin (135вћ¦) = 2.12 cm.
2.3. NEWTON’S LAWS
2.3
31
Newton’s Laws
Armed with vectors, let’s consider Newton’s laws again. If N different forces act on an object,
then Newton’s equation is Fnet = ma , where the net force
N
Fj в‰Ў F1 + В· В· В· + FN
Fnet =
j=1
is equal to the vector sum of all forces acting on the object. Thus, when you have multiple
forces on a single object, they add according to the rules of vector addition. In this lab, we
will only consider an object at rest (in equilibrium), even though several forces act on it. This
means that the net force
Fnet = 0
(2.3.1)
So, in all our experiments in Lab. 2, the combinations of forces should add up to zero.
Let’s take a moment to look at the units of force. We know that Fnet = ma, so force must
have units kgm/s2 using S.I. units. This combination of fundamental units is called the Newton.
One Newton of force has the value 1 N = 1 kgВ·m/s2 .
2.4
The Force Table
Figure 2.3: The force table.
A force table is a table that allows masses to hang off the sides on pulleys. The hanging
masses add tension T to the string. The string then exerts a force, with magnitude T , on each
object that it is attached to. The direction of the force is tangent to the string at the point of
contact. In these experiments, the strings all attach to a ring in the center.
When the forces exerted by the strings add up to zero (vectorially), there is no net force,
and the ring in the center will stay in the center. When there is a small net force, the ring will
show a tendency to move/stay in one direction when tapped. The friction in the pulleys makes
very small forces undetectable. If there is a fairly large net force on the ring when it is at the
center, it will be pulled to one side and to a position where the net force is zero.
32
LAB 2. NEWTON’S LAWS AND VECTOR ADDITION
Free Body Diagrams
A useful way to visualize the forces on an object is to draw a free body diagram. One way to
construct a free body diagram is to draw all the vector forces on an object so the tails of the
vectors lie at the same spot. Note that this is not a vector addition; you cannot measure a sum
from this plot. Here’s an example of a mass with three forces:
90вћ¦
120вћ¦
150вћ¦
3N
@
60вћ¦
30вћ¦
13
5
вћ¦
2 N @ 0вћ¦
180вћ¦
3 N @ 270вћ¦
210вћ¦
240вћ¦
270вћ¦
0вћ¦
330вћ¦
300вћ¦
Figure 2.4: A free body diagram.
Food for thought
This example does not add up to 0. What is the resultant? What will the object
do?
Notation Corner
There were three concepts introduced here:
Algebric addition: Adding vectors by adding components.
Graphical addition: Adding vectors by placing them tip to tail and measureing.
Free body diagram: A way to visualize vectors. You cannot use this to add
vectors.
Note that vectors can only be added using one of the discribed methods. You cannot add vectors by adding magnitudes and angles directly.
Lab 2
Notes
This is for initial comments, work, etc. that you may have from the reading and the prelab
videos. Put down anything you think might help for the upcoming lab.
33
Lab 2
Guidelines
Raw Data
Two String
Attach two masses on to the ring (via strings). Hang them off at 0вћ¦ and at 180вћ¦. The 0 degree
mass should be 200 g, including mass hanger. How much mass do you have to add (1 g at a
time) to one of the masses and to get the ring to move when tapped (no longer in equilibrium)?.
Write down the actual measurements1 of the masses you used. Subtract to find the maximum
extra mass you can add while still keeping the system in equilibrium.
Three Strings
You are given two masses and angles for the three strings. Try to calculate algebraically and
graphically what the remaining mass and angle should be (calculation), then test it in lab
(measured). Remember that you want to find the sum, and then put a force equal and opposite
to make the final sum 0. Note: to get an opposite vector, add 180вћ¦ to the angle.
Test your values on the force table and try to observe equilibrium. Tap the ring to make sure
it does not move. Write down measured masses.
Draw a force diagram, too.
Four Strings
On this one, you will not precalculate the values. Just experiment, and see if you can get
the ring to look like it is in equilibrium. You’ll test the lab answer later. Ask your TA to fill in
the TA blank for your group before starting.
Find the resultant algebraically and graphically. You will be summing four vectors instead of
two.
You probably missed true equilibrium by a little. Let’s see how close you got. First, how far
off do you think you were (maximum force error)? Remember, there are four pulleys, you can
use the results of the two string experiment.
1
Use triple beam balance
34
Lab 2
Worksheet
Name:
Table:
Partner(s):
Two Strings
Angle
Measured m (g)
0вћ¦
180вћ¦
Maximum extra mass:
Maximum extra force (carefully convert to N): ОґF2 =
Maximum extra force per pulley: ОґF
Three Strings
Three String Masses, Calculation
Remember: sum the components!
Name
Mass(g)
Direction
First
150 g
0вћ¦
Second
150 g
90вћ¦
Total (sum):
Expected vector:
вћ’вћ’вћ’
Magnitude (N)
x-comp (N)
y-comp (N)
вћ’вћ’вћ’
вћ’вћ’вћ’
Three String Masses, Measured
Angle
Expected m (g)
0вћ¦
150 g
90вћ¦
150 g
вћ’вћ’вћ’
35
Measured m (g)
36
Lab 2. Worksheet
Three String Graphical
Make vectors as large as possible! Must fill over half the grid. Add forces from first and second
strings to get the sum. Make sure you include arrow heads to indicate direction. Always report
measured magnitude and direction for any vector sum.
Scale:
Sum:
Lab 2. Worksheet
37
Force Diagrams (3 string, 4 string)
120вћ¦
90вћ¦
60вћ¦
120вћ¦
150вћ¦
30вћ¦
180вћ¦
0вћ¦
210вћ¦
300вћ¦
270вћ¦
30вћ¦
180вћ¦
330вћ¦
240вћ¦
60вћ¦
150вћ¦
0вћ¦
210вћ¦
90вћ¦
330вћ¦
240вћ¦
270вћ¦
300вћ¦
Four Strings
Angle
Expected m (g)
0вћ¦
100
75вћ¦
100
Measured m (g)
вћ’вћ’вћ’
TA:
вћ’вћ’вћ’
Error
Maximum force error, 4 pullies: ОґF4
How did you calculate this?
Four String Algebraic Resultant
Name
Direction
First
0вћ¦
Second
75вћ¦
Magnitude (N)
x-comp (N)
y-comp (N)
Third
Fourth
Resultant (sum):
How did the magnitude of the resultant compare with the maximum error? (Smaller/Larger)
38
Lab 2. Worksheet
Four String Graphical Resultant
Remember there are 5 vectors on this graph! Four measured vectors and a resultant (sum)
vector.
Scale:
Sum:
Include postlab
TA sign-off:
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