Download 15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation
Energy Conversion
Energy can be converted from one form to another.
This happens constantly
Batteries convert chemical energy to electrical energy, then the light bulb converts the electrical
energy to electromagnetic energy (light) and thermal energy.
I convert kinetic energy to potential energy by pulling back the “pull back” toy, then the potential
energy is converted back to kinetic energy when I let it go.
I convert kinetic E to gravitational potential E when I lift a ball into the air. The potential E is
converted back into kinetic E when I drop the ball.
Energy conversion of a match:
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Chemical E from muscles
Kinetic E of arm moving
Friction between match and rough surface converts KE to thermal
Thermal E triggers chemical reaction at the tip of the match releasing chemical E.
Chemical E converted to thermal E and electromagnetic E (light).
Conservation of Energy – Energy cannot be created or destroyed. *
When energy changes from one form to another, the total energy stays the same.
Many things appear to be losing energy, but actually, friction causes kinetic E to be converted to
thermal E. (friction with the ground, and with the air)
All moving parts are subject to friction. Friction is a major cause of energy consumption (use) in
cars and factories.
*Energy and Mass
Einstein’s equation, E = mc2, says that energy and mass are equivalent (equal) and can be converted
into each other. (c is the speed of light: 3 x 108 m/s)
Energy is released as matter is destroyed – which is what happens in nuclear reactions (both fusion
and fission). Matter can be created from energy.
Since c2 is such an incredibly large number, a tiny amount of matter can produce an enormous
amount of energy.
The law of conservation of energy has been modified to say that mass and energy together are
conserved.
15.1 and 15.2 Math problems
On a separate piece of paper, solve the following problems. Show all work –
including info list, formula, setup, and answer with correct units.
KE = ½ mv2
gravitational PE = mgh
v = √(2 • KE) / m
Mechanical energy = KE + PE
m = 2 • KE
v2
g = 9.8 m/s2
conservation of mechanical energy:
(KE +PE)beginning = (KE + PE)end
1. Calculate the potential energy of a 45 kg diver on a 10.0 meter platform.
2. A 70.0 kg man is walking at a speed of 2.0 m/s. What is his kinetic energy?
3. A 1400 kg car is movng at a speed of 25 m/s. How much kinetic energy does the car have?
4. A 50 kg cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running?
5. A 60kg person walks from t ground to the roof of a 74.8 m tall building. How much
gravitational potential energy does she have at the top of the building?
6. A 10 kg rock is dropped and hits the ground at a speed of 60 m/s. Calculate the gravitational
potential energy of the rock before it was dropped.
7. A diver with a mass of 70 kg stands motionless at the top of a 3.0 m high diving platform.
Calculate his potential energy relative to the water surface while he is standing on the
platform, and his speed when he enters the pool. (Assume the diver’s initial vertical speed
after diving is zero.)
8. A pendulum with a 1.0 kg weight is set in motion from a position 0.04 m above the lowest
point on the path of the weight. What is the kinetic energy of the pendulum at the lowest
point? (Assume there is no friction.)
Numbers 9 and 10 are not math problems
9. Read p.456 in your book. Write the process of energy conversion of a pendulum.
10. Read p.457 in your book. Write the process of energy conversion of a pole vault.