PES 1110 Fall 2013, Spendier
Lecture 1/Page 1
Lecture 1/ Chapter 1/ Measurements
Lecture today:
1) Introduction
2) Chapter 1
3) Short math Skills test (not graded): purpose is to help me gage your math
background to efficiently present the material in this course. A language of
physics is math, so it is important to know the math.
First HW will be assigned this Wednesday and due in one week – on Wednesday
after Labor Day holiday. First quiz will be a week from this Friday on the material
covered in the first 4 lectures.
Why do you take this class? What is Physics?
So why are you taking this class? Is it just a requirement? Do you think this course is
relevant to your future carrier? You might ask yourself: What has any of this got to do
with my life? This course will use the concepts of physics, from which we will draw
valid conclusions about the real world and therefore learn about the fundamentals from
which all scientific and engineering applications spring.
Reasons not to study:
1) old problems
2) we know the answer
3) problems are not interesting
Reasons to study:
old put important: Galileo and pendulum
In 1583 at the age of 19, Galileo was sitting in the Pisa cathedral. He noticed s
chandelier swinging and observed that the time to complete the total motion (one
period) is independent of how far the pendulum swings - this is the basis for
clocks.
our quest to break records: How could Felix Baumgartner break the sound
barrier? On Oct. 14 2012 Felix reached an estimated speed of 833.9 mph (372 .8
m/s), surpassing the speed of sound. 690 miles per hour ( 308 m/s). What is the
terminal velocity (the point at which a falling object stops accelerating ) for
normal skydivers? 195 km/h (122 mph or 54 m/s). Terminal velocity, a concept
familiar to skydivers, refers to the point at which a falling object stops
accelerating. Drag, or resistance, is one of the key factors causing terminal
velocity. Bailing out at a very high altitude (jumped from 128,000 feet or 39 km),
where the air is thin, enabled Felix to break the speed of sound before reaching
more dense air that will create drag and eventually result in his terminal velocity.
http://www.youtube.com/watch?v=VKojXTWJIhg
PES 1110 Fall 2013, Spendier
Lecture 1/Page 2
Measuring things
Science and engineering is based on observation, specifically measurements and
comparison. This is why we call physics an empirical science. Measurement is one of the
foundations of science. Scientists use measurements as part of the observation and
experimental parts of the scientific method. When sharing measurements, a standard is
needed to help other scientists reproduce the results of an experiment. We need to be able
to compare a measurement with another measurement.
S.I. units: http://www.youtube.com/watch?v=83e3n83Re5s
It comes from French: Le Système international d’unitès
The International System of Units or "SI Units" is a standard set of units agreed on by the
scientific community. This system of measurements is also commonly called the metric
system, but SI units are actually based on the older metric system. The names of the units
are the same as the metric system, but the SI units are based on different standards.
There are seven base units that form the foundation of the SI standards.
Quantity
Length *
mass*
time*
temperature
electric current
amount of substance
luminous intensity
Unit
meter
kilogram
second
Kelvin
ampere
mole
candela
symbol
m
kg
s
K
A
mol
cd
* Important ones for us in this course
We will use the SI units in this class. However, the whole point of units is that you can
choose whatever units are most convenient, such as:
%%%%%%%%% aside – you don’t need to know this
Avogadro's number - chemists use Avogadro's number to mean 6.0221417930 x 1023,
the number of atoms or molecules in one mole. It was named after Italian scientist
Amedeo Avogadro.
Reduced mass of the electron as a unit of mass in atomic physics.
Fun and unusual ones:
- baker's dozen - If you buy a dozen loaves of bread, bakers usually throw one in
for free, so baker's dozen means 13. They didn't do this out of the goodness of
their heart: the practice came to be in the 13th century, when a medieval English
law made it so a baker could be punished by chopping his hand off with an axe if
he was found to be shortchanging a customer. Tossing in an extra loaf of bread
seemed to be a prudent way of keeping one's hand.
PES 1110 Fall 2013, Spendier
Lecture 1/Page 3
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moment - If you ask someone to wait a moment, you're asking them to wait for a
very short period of time. But how short? Turns out a moment is a medieval unit
of time equals to 1/40th of an hour or 1.5 minutes.
- Scoville heat units - this unit measures the hotness of a chili pepper. A scoville is
the dilution factor of a solution of chili pepper extract until the "heat" (the amount
of the chemical capsaicin) is no longer detectable to tasters. A bell pepper has a
Scoville rating of 0, whereas a habanero has a rating of 200,000 (meaning a
solution of habanero extract needs to be diluted 1:200,000 before the heat goes
away). Grown in new Mexico, the Trinidad Scorpion Butch T variety pepper is
one of the hottest pepper in the world with 1,463,700 Scoville. A pepper spray is
rated between 2 and 5.3 million Scoville.
%%%%%%%%% end aside
Derived SI Units:
All other quantities can be derived from these units. We will soon talk about speed, i.e.
velocity which is measured in m/s – combination of 2 SI units. These units are called
derived units.
Examples 1: Area and volume are two derived units using length in meters (1 SI unit):
Area = (length) (length) = m2
Volume = (length) (length) (length) = m3
Example 2: another derived unit is volume (you will need this for your HW)
cube of 10 by 10 by 10 centimeters has a volume of one liter
Check Units:
Besides being an essential part of any answer, units can help you to make sure that your
answer is correct: both sides of an equation must have the same units.
Some unit conversions:
1 kg = 1000 g = 2.20 lb = 35.27 ounces
1 m = 100 cm = 3.28 ft
1 inch = 2.54 cm
1 hour = 60 min
1 min = 60 sec
Changing units (unit conversion):
Sometimes it is useful to change the units. We change units by using a conversion factor.
Conversion factor = a ratio of units that is equal to unity
Example 1:
1 min = 60 s
1 hour = 60 min
Hence appropriate conversion factors are
1 hour
60 s
= 1 and
=1
60 min
1 min
PES 1110 Fall 2013, Spendier
Lecture 1/Page 4
So we can now ask, how many minutes are in two hours?
 60 min 
2 hour = 2 hour (1) = 2 hour 
= 120 min
1 hour 
Example 2:
How many people does it take to drag a big Stonehenge rock (50 tons)
assumptions:
1 person = 70 kg
each person can drag his/her weight
1 ton = 1000 kg
So we end up with a unit problem:
1000 kg 1 person
50 tons ×
×
= 571.4 persons = 571 persons
1 ton
70 kg
The only way this gets a bit trickier is in the HW problems, if you start converting areas
or volumes. What is a square inch? An inch times an inch right? So one side is 2.54 cm
and the other side is 2.54 cm – so to convert my areas I also have to square my
conversion factors. 1 square inch is (2.54)2 cm2 once you realize that you are good to go.
Estimation:
When making a measurement, results are NEVER exact.
A visitor to the Royal Tyrrell Museum was admiring a Tyrannosaurus fossil, and asked a
nearby museum employee how old it was. "That skeleton's sixty-five million and three
years, two months and eighteen days old," the employee replied. "How can you know it
that well?" she asked. "Well, when I started working here, I asked a scientist the exact
same question, and he said it was sixty-five million years old – and that was three years,
two months and eighteen days ago."
Lots of digits typically mean a very precise measurement. But we are not always
interested in that much precision. We typically use the number of significant figures to
show the amount of precision in a measurement. The more significant figures in a
measurement, the more precise the measurement.
Significant Figures
Or, we can express them implicitly by using the correct number of significant figures:
A measurement is made with the result 2.94 cm.
The implicit uncertainty is 0.01 cm.
A measurement is made with the result 0.0054 s
The implicit uncertainty is 0.0001 s.
PES 1110 Fall 2013, Spendier
Lecture 1/Page 5
Leading and sometimes trailing zeros are not considered significant:
0.0054 has only two significant figures.
78100 has only three significant figures.
78100.00 has seven significant figures. (due to “.”)
Example:
length A = 3.467 km and length B = 1.046 km. What is A + B to 2 Sig.Fig?
Here we add first and then round:
A + B = 3.467 km + 1.046 km = 4.513 km = 4.5 km
NOTE: In this course will usually give numerical values with three significant
figures
Scientific Notation
It is easier to see how many significant figures there are when written in scientific
notation:
0.0054 = 5.4 x 10-3
78100 = 7.81 x 103
Metric Unit Prefixes
We see that big and small numbers are not good to write. That is why we use scientific
notation. SI units can be expressed by powers of 10 using metric prefixes. These prefixes
are commonly used instead of writing very large or very small numbers of base units.
kilo (k) = 1000 = 103 (1 km is covered in a 10 min walk)
mega (M) = 106
giga (G) = 109
tera (T) = 1012 (The US is tera Dollars in dept. That is $100 trillion dollars in debt.)
centi (c) = 0.01 = 10−2 (1 cm is the diameter your little finger)
mili (m) = 0.001 = 10−3 (1 mm is the diameter of a ballpoint pen)
micro (µ) = 10−6 ( 1µm size of cells and bacteria)
nano (n) = 10−9 ( 1 ns is the time for light to travel 0.3 m)
pico (p) = 10−12
femto (f) = 10−15
atto (a) = 10−15 ( the shortest laser pulse is just 67 attoseconds)
PES 1110 Fall 2013, Spendier
Lecture 1/Page 6
%%%%%%%%% aside – you don’t need to know this
As a Biophysicist, I work at length scales of micro meters = 10-6m and
nano meters = 10-9m
googol - The googol was invented in 1938 by mathematician Edward Kasner, who asked
his then 8-year-old nephew Milton Sirotta what he would name a really, really, really
large number. A googol is a large number indeed: it is 1 followed by 100 zeroes or 10100.
If googol sounds familiar, that's because Larry Page and Sergey Brin named their
company Google based on this word which was picked to signify that the search engine
was intended to provide large quantities of information. They even called the Google
headquarters in Mountain View, California, the Googleplex.
%%%%%%%%% end aside