Transcript Welcome to Physics 112N - Physics, Computer Science and

Welcome to Physics 151
Adapted from Hyde-Wright, ODU
©James Walker, Physics, 2nd Ed. Prentice Hall
Preliminary list of topics
Walker, Chapters 1-18
• Description of motion: Kinematics
– Position (in 1-, 2-, 3- dimensions), Velocity & Acceleration
• Forces and Motion: Net Force equals mass times acceleration
• Examples of forces:
– Contact forces (friction, “normal” force)
– Gravity
– Elastic forces: Springs and waves
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Kinetic Energy, Potential Energy, Work
Momentum, Collisions
Torque and Statics
Oscillations & Waves
Fluids & Thermodynamics
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Introduction
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Physics and Laws of Nature
Units of Length, Mass, and Time
Dimensional Analysis
Significant Figures
Converting Units
Precision and Significant Figures
Problem Solving in Physics
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What is Physics?
• In science in general, and physics in particular, we seek to
develop and evaluate theories/concepts/ideas/perceptions....
• The goal is to gain a deeper understanding of the world in
which we live.
• Physics is the study of the fundamental laws of nature, which
are the laws that underlie all physical phenomena in the
universe.
• These laws can be expressed in terms of mathematical
equations.
• We can make quantitative comparisons between the
predictions of theory and the observations of experiment.
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Why Physics?
• 19th century: economy was dominated by
harnessing the power of steam.
• 20th century economy was dominated by the
internal combustion engine and the microelectronics revolution.
• 21st century economy may be revolutionized
by the manipulation of individual atoms.
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Distance
• It is impossible to talk about distance or motion in any
terms other than quantitative.
– How far is it from CNU to Colonial Williamsburg?
– How far is it from the Earth to the nearest large galaxy?
• It is impossible to speak quantitatively without defining a
unit of measure.
– It is meaningless to say that the distance to the Colonial
Williamsburg is 20.
• Physical quantities have both numerical value and units.
It is 20 miles from CNU to Colonial Williamsburg.
20 miles equals 32 kilometers.
20 miles also equals 105,600 ft = 32,208 m.
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Typical Distances
Distance from the Earth to the nearest large galaxy (the
Andromeda Galaxy, M31)
2 x 1022 m
Diameter of our galaxy (the Milky Way)
8 x 1020 m
Distance from the Earth to the nearest star (other than the Sun)
4 x 1016 m
One light year
9.46 x 1015 m
Average radius of Pluto’s orbit
6 x 1012 m
Distance from Earth to the Sun
1.5 x 1011 m
Radius of Earth
6.37 x 106 m
Length of football field
102 m
Height of a person
2m
Diameter of a CD
0.12 m
Diameter of the aorta
0.018 m
Diameter of the period in a sentence
5 x 10–4 m
Diameter of a red blood cell
8 x 10–6 m
Diameter of the hydrogen atom
10–10 m
Diameter of a proton
2 x 10–15 m
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Time
• The second is defined as the duration of 9,192,631,770 oscillations of
a particular atomic transition in Cesium-133 (defined by the spectral
color of the light).
– It is not an accident that the typical human heartbeat is 1 sec.
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Age of the universe
5 x 1017 s
Age of the Earth
1.3 x 1017 s
Existence of human species
6 x 1013 s
Human lifetime
2 x 109 s
One year
3 x 107 s
One day
8.6 x 104 s
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The Equals Sign
• At the heart of mathematical notation is the equals sign "=".
• In everyday language, "equals" means "is the same thing".
• In algebraic notation, "=" means "has the value of"
– v = 60 mi/hr, means “my speed on the highway, represented by the
symbol ‘v’ has a value of 60 mi/hr”
– F = m a, means “The net force acting on an object is numerically equal to
the mass ‘m’ of the object times its acceleration ‘a’. In this example the
left and right hand sides of the equal refer to totally different concepts,
that Newton tells us nonetheless have the same numerical values.
• Notice that the symbol (e.g. ‘v’) includes both magnitude and units.
– We do not write v mi/hr, nor v = 60,
– but v = (60 mi/hr)
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Mass
• In SI units, mass is measured in Kg. We don’t define the Kg to be the
weight but rather the ‘amount’ of substance contained in the object,
which is an intrinsic and unchanging property.
• Weight, in contrast, is the measure of the gravitational force acting on
an object. W = mg
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Galaxy (Milky Way)
Sun
4 x 1041 kg
2 x 1030 kg
Earth
5.95 x 1024 kg
Elephant
5400 kg
Human
70 kg
Honey bee
1.5 x 10-4 kg
Bacterium
10-15 kg
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Converting Units
• How do we convert from inches to cm, or miles to
km (without crashing into Mars as NASA recently
did!).
• By international convention 1 in = 2.54 cm
(4.75in )  (4.75in )
2.54cm
in
 (4.75  2.54)cm  12.065cm
1in
in
Notice that units divide just like numbers (in/in) =1
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Experimental Uncertainties
• Physics is quantitative, not exact.
• Every physical quantity has an associated uncertainty,
either in its measurement or in its prediction.
– Note that ‘error’ is used sometimes to mean uncertainty and
sometimes to mean mistake.
– Carpenters rulers are marked off in 1/8 inch increments. Given the
flexibility of a wooden house it is unnecessary and useless to try to
measure dimensions in home construction to better than 1/8 inch
– Estimating the uncertainty is often the most difficult part of a
measurement. In your physics lab, you must make judgments
about the precision of the distances, times, weights, etc. that you
measure.
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Precision and Significant Figures
• If we say the acceleration due to gravity is 9.80 m/s/s,
(three significant figures) we imply an uncertainty of +/0.01
 Thus: 9.80 m/s/s = (9.80 ±0.01) m/s/s
 9.80 m/s/s is not the same as 9.8 m/s/s
• In the homework and tests, we will assume 3 significant
figures unless specified otherwise.
– To avoid round-off errors, you must carry through all intermediate
calculations to at least 4 significant figures.
– Use of excessive significant figures will result in point deductions
on tests. Even though your calculator gives answers to 10
significant figures, it is wrong to write, e.g. “the ball stayed in the
air for 2.314078504 sec” if all the input data are not given with that
precision.
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