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http//www.nearingzero.net (work059.jpg)
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First Four Weeks Schedule
Jan. 23 Climate Changes Video (104 Phys.) Jan. 25 Video Discussion Force and Motion
Jan. 30 Force and Motion Feb. 1 Force and Motion
Feb. 6 Global Dumping Ground Video (Phys) Feb. 8 Video Discussion Energy
Feb. 13 Ozone Depletion Video (104 Phys.) Feb. 15 Video Discussion Energy Ozone
Lab meets Friday!
The body is most fully developed from thirty to
thirty-five years of age, the mind at about
forty-nine.Aristotle
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Environmental Issues Issue 10 Automobiles II
If you choose this Issue, youll explore
automobile trans-portation trends in China.
Youll consider scenarios that might occur if
per-capita motor vehicle use approaches that in
the US.
Issue 10 is rather short, but it is sufficiently
detailed that it is all you would need to do.
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I need to revisit the subject of mass for a bit
Here's something to think about. How do you
measure mass?
I expect most answers will involve some kind of a
balance or scale. Spring scales use a spring to
balance your weight. Would a spring scale work on
the moon?
I expect most answers will involve some kind of a
balance or scale. Spring scales use a spring to
balance your weight. Would a spring scale work on
the moon? Yes, but not accurately.
Please visit howstuffworks.com to see how lots of
stuff works.
Would a spring scale work in outer space?
Would a spring scale work in outer space? No.
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Some of you may have thought of a balance scale.
The simplest kind works like the one you saw in
the first lecture
Two identical pans, identical distances from
hanging point. Put unknown mass in one pan,
known masses in the other pan until balance is
achieved.
Triple beam balances are more complex, and are
based on ideas the physics 8 lab students
experienced in the first lab, but still rely on
balancing known masses against unknown masses.
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Would either of these balances work on the moon?
Yes, and accurately.
Would either of these balances work in outer
space?
No! Why not?
How would you measure mass in outer space? You'd
have to use Newton's second law and some device
that measures the acceleration of an unknown mass
as a result of a known force.
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Such an instrument is called an inertial balance,
and works in space.
Two NASA inertial balance designs (the pictures
were live links).
This brings up an interesting question is the
mass that responds to gravity the same as the
mass that accelerates in response to forces?
In the previous we investigated the difference
between inertial and gravitational mass, and
concluded there is currently no evidence to
suggest they are different.
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All those who are non-science majors, move to the
front three rows of the room, and all those who
are science majors, move to the back of the room.

Science majors take a little break.
Non-science majors after a few minutes of
discussion, give me a technical term in your one
of your fields that you would like to have the
science majors attempt to define.
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There are two more subjects I want to cover in
this discussion of forces and motion momentum
and density.
What does momentum mean to you?
Guess what? Non-science majors get to write your
definition of momentum on a piece of paper and
turn it in for todays attendance.
Science majors, please define the term that the
non-science majors have chosen. Turn in your
definition for todays attendance.
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Momentum is related to the "mass in motion part"
of Newton's first law. Remember, that law says
an object in motion remains in motion unless
acted on by external forces. Momentum is a
measure of a body's "desire" to remain in motion.
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In physics, momentum is calculated using the
equation pmv. (I guess since m was already used
for mass, "p" is as good a symbol as any for
momentum.) The more mass you have, or the faster
you are going, the greater your momentum.
There is an important law of physics, known as
the Law of Conservation of Momentum. This law
says that momentum is conserved in any process,
as long as no external forces are acting.
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If your car and an 18-wheeler collide, what does
the Law of Conservation of Momentum have to say?
graphics from http//www.glenbrook.k12.il.us/gbssc
i/phys/mmedia/
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It has been observed experimentally and verified
over and over that in the absence of external
forces, the total momentum of a system remains
constant.
The above is a verbal expression of the Law of
Conservation of Momentum.
It sounds like an experimental observation, which
it is
which implies maybe we just havent done careful
enough experiments, and that maybe some day we
will find the law is not true after all.
But the Law of Conservation of Momentum is much
more fundamental than just an experimental
observation.
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In 1905, mathematician Emmy Noether proved the
following theorem
For every continuous symmetry of the laws of
physics, there must exist a conservation law.
For every conservation law, there must exist a
continuous symmetry.
The laws of physics are invariant under
coordinate transformations.
Thats a fancy way of saying you and I can pick
different coordinate systems for measurements and
experiments, but we will still arrive at
equivalent results.
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If the laws of physics were not invariant under
coordinate transformations, then everybody would
have his own version of the laws of physics.
Not much point in doing physics, if thats true!
The Law of Conservation of Momentum is a
mathematical consequence of the invariance of the
laws of physics under coordinate transformations.
Any violation of the Law of Conservation of
Momentum would be as revolutionary (if not more
so) as Einsteins relativity.
But even if a violation were found, any new
laws of physics would contain all our old ones,
which would still work under normal
circumstances.
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Conservation laws are fundamental, powerful, and
beautiful.
We will see another one soon.
In contrast, Newtons laws work only in the
macroscopic world, and are only an approximate
description of nature.
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Before I leave momentum, have you ever thought of
racing a train across the crossing?
What is your cars mass? How far does it take you
to stop your car when you are going 60 miles per
hour?
Some of the distance traveled is due to your
reaction time, and the rest is due to the time
needed from the frictional forces applied by your
brakes to "use up" your car's momentum.
What is a trains mass? How far does a train
traveling at 60 miles per hour go before the
friction forces from its brakes cause it to stop?
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Density
First, a little experiment
Please make for me a cubic meter of air.
No peeking at the next slide!
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Air has a density of about 1.3 kilograms per
cubic meter. That much air weighs about 0.6
pounds (near the surface of the earth).
Air is not very dense, but I still am surprised
at how much a cubic meter of air weighs.
The density of water is about 1 gram per cubic
centimeter. It takes 100 centimeters to make a
meter, so it takes 100x100x100106 cubic
centimeters to make a cubic meter.
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A cubic meter of water has a mass of 106 grams,
or 103 kilograms, or 1000 kilograms.
An object having a mass of 2.2 kilograms weighs a
pound at the surface of the earth, so the cubic
meter of water weighs about 454 pounds.
Ive been using the word density without
defining the term, so
What happens when you put something in water.
Well, yes, it either sinks or floats but the
water also pushes on it. Jump in the pool and
feel the water trying to push you out.
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Who remembers the story of Archimedes and the
kings crown?
http//www.engineering.usu.edu/jrestate/workshop/b
uoyancy.htm
As the story goes, the king of Syracuse had
given a craftsman a certain amount of gold to be
made into an exquisite crown.
If Archimedes was born and raised in Syracuse,
Sicily, why do we consider him a Greek?
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When the project was completed, a rumor surfaced
that the craftsman had substituted a quantity of
silver for an equivalent amount of gold, thereby
devaluing the crown and defrauding the king.
Archimedes was tasked with determining if the
crown was pure gold or not. The Roman architect
Vitruvious relates the story
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While Archimedes was considering the matter, he
happened to go to the baths. When he went down
into the bathing pool he observed that the amount
of water which flowed outside the pool was equal
to the amount of his body that was immersed.
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Since this fact indicated the method of
explaining the case, he did not linger, but moved
with delight, he leapt out of the pool, and going
home naked, cried aloud that he had found exactly
what he was seeking.
For as he ran he shouted in Greek Eureka!
Eureka! (Eureka translated is "I have found it").
Although there is speculation as to the
authenticity of this story, it remains famous.
Probably no other tale in all of science
combines the elements of brilliance and bareness
quite so effectively.
Whether the story is true or not, there is no
doubt to the truth of Archimedes understanding of
buoyancy.
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Archimedes death also makes an interesting story
In 212 BC Syracuse surrendered to Rome. Before
sending his men to sack the city Marcellus told
them Spare that mathematician. Plutarch records
what happened next
As fate would have it, intent upon working out
some problem by a diagram, and having fixed his
mind alike and his eyes upon the subject of his
speculation, he Archimedes never noticed the
incursion of the Romans, nor that the city was
taken.
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In this transport of study and contemplation, a
soldier, unexpectedly coming up to him, commanded
him to follow Marcellus which he declining to do
before he had worked out his problem to a
demonstration, the soldier, enraged, drew his
sword and ran him through.
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Archimedes is remembered by most of us as a
mathematician, but he also invented fabulous war
machines.
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News Flash!
Archimedes invents Death Ray that sets enemy
ships on fire!
My Physics 24 text and Mythbusters say its
impossible!
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News Flash!
MIT students set wooden ships on fire with death
rays! Details here!
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Archimedes also overcame our 34-foot straw
problem (remember the last lecture) by inventing
the Screw of Archimedes.
www.nearingzero.net
What is not so well known about Archimedes is
that he had a career record of 40 wins and 28
losses while pitching for the Cosmic Ionians. For
proof, see here.
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Finally, you can go here to read why the
traditional story of Archimedes and the kings
crown is probably not true
http//www.mcs.drexel.edu/crorres/Archimedes/Crow
n/CrownIntro.html
What is important for us, and what Archimedes
understood, is that an object immersed in a fluid
experiences a buoyant force equal in magnitude to
the weight of the fluid displaced.
Archimedes could have done this experiment
The craftsman lives.
The craftsman dies.