Download Stacey Carpenter - University of Hawaii

PPT
Energy
Developer Notes:
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Physics
work
Physiology
ability to do
work
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potential
energy
kinetic
energy
fat
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motion
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calories
840967898
calories
diet
metabolism
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Delivery
We've seen that Ft = ∆(mv). What about Fd? We've seen it as in
levers, and in torque.
Activity - dynamics carts (can we use anything else?) hanging and
on various slopes. Find the force and the distance and see if they're
all the same. (Acceleration at the beginning is equaled by the
deceleration at the end.)
Fd is work. Fd = mad = work. Work is mad. Fd is measured in
Nm, which is also called J (Joules), which is kgm2/s2.
Appliance cart over curb or up ramp?
No matter which way you got it there, at the end you still have the
same force at the same height, able to do the same work.
So now you have Fd. What is that? You can do things with it, like
water above a waterfall or dam - water mills, saws, generators. Pile
driver. Bucket of water over the doorway. (Can't tell the students
about some of these.) Bike on top of a hill. Counter-weight on an
elevator. Gravity-driven is the easiest to visualize.
Takes work to get it up there, but now it can work for you.
Fd is potential energy, in J. mad = kg(m/s2)m = kgm2/s2
So what happens when something falls from a height? When it gets
to the bottom, we don't necessarily know where it came from. It has
mass, it has velocity. At the moment it strikes, it doesn't really
matter where it started. If it was shot at the same speed, who cares?
So, how does that relate to its PE?
The mathematical derivation is Fd = mad. d = at2/2. Substitute
for d and it = ma2t2. a2 is (d/t2)2, and a t2 cancels, leaving
m(d/t)2/2 = mv2/2. So, the PE turns into moving, or kinetic,
energy. PE = KE.
KE is also in J. kg(m/s)2 = kgm2/s2.
It's nice to get it to m and v only, not a, because then you can relate
things that are going a steady speed and see what energy they have,
not just objects that are accelerating.
Do some exercises with things falling at the acceleration of gravity.
Activity - Projectile prediction with PE -> KE -> v, get time from
the height to the floor (t = √(2d/a), and d = vt.
Calories are another measure of energy, like Joules. The potential
to do work.
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PPT
Physics
power
conservation
of energy
Energy
Physiology
strength
nutrition
energy in,
energy out
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heat
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heat vs.
temperature
regulatory
system?
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840967898
Delivery
So you can do some work, like dig a hole. Remember John Henry
(can I get a recording? Lord forbid I sing!)? His was a contest of
power. They wanted to dig a tunnel, and they were checking who
was faster, a man or a machine. John Henry won, but he died doing
it - the machine won in the long run. The question was what can do
the same amount of work in less time. That's power.
Power is work/t, J/s. Horsepower.
There's more to life than calories.
This is like conservation of momentum. There's only so much
energy in the universe.
1st Law - conservation of energy. This is the biggy.
∆U = Q - W, The change in internal energy (U) of a closed system
equals the heat (Q) added to the system minus the work (W) done
BY the system. (But I don't totally understand this.)
Total Energy = KE + PE + Q For mechanical systems.
Exercises - ?
Activities - ?
Heat is a measure of the total amount of kinetic energy in
something.
activity - rub your hands together - they get warm. You need to
apply a force because of the friction. So where does the work
energy go? To heat. How about a car stopping? Where does its
kinetic energy go? To heat. How about on a razor scooter? Do you
ever step on the brake with bare feet? Heat is a by-product of work.
activity / example
Temperature is the average kinetic energy of the molecules in
something.
Heat and temperature are different, like mass and weight differ.
Heat is the total kinetic energy, measured in Joules. Temperature is
how hot something is, the average (not total) kinetic energy,
measured in degrees,.
activity / example - a bag of ice cubes vs. one.
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PPT
Physics
entropy
Energy
Physiology
death?
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maintenance
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energy
840967898
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Delivery
2 Law - entropy. It ends up as heat. A difference makes a
difference. Can't do anything without a difference. Difference is
information.
∆S = Q / T, The change in entropy (S) of an object equals the
amount of heat added divided by the absolute temperature (Kelvin).
Activity - guess how I want these blocks put together. Assemble
this electronic circuit so it works. Throw these dice until they all
form a neat square. Throw the dice until they equal 12. Build a
sand castle and see what it looks like tomorrow - better?
Activity - Are there any heat related activities? Ice in a glass - it
melts and it all turns to the same temperature - it's now useless to
cool your drink. The difference in heat that drives a car engine,
steam engine.
Lecture - vandalism, destruction, 9/11, war, drop the clock on the
wall, step on a piece of chalk - it's easier to break things than make
them work. There's no pride in it. We're in a constant battle against
entropy. It takes energy to put things together in a particular way there are many more ways that are wrong.
Global energy
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