Download Energy Unit Concept Review Questions

Energy Unit Review Questions
Work and Power
1. How much Work does a dog do while pulling with 50 N on a sled for 200 m?
10, 000 J
2. If we know the amount of Work done by the dog in the first question, how does that relate to the change in
energy of the sled?
The amount of work is equal to the change in kinetic energy. So, the change in kinetic energy is 10, 000 J.
3. How is Work related to Force?
Work = Force * Distance
If force increases, work increases. If force decreases, work decreases.
4. How is work related to energy? If I do more work do I use more or less energy?
Since work is the change in kinetic energy according to the work energy theorem, the change in energy will
affect the amount of work done. If more energy is used, more work will be done.
5. Jessica and Jeff each push an identical block across a frozen, smooth lake. Jeff pushes with a Force of 60
Newtons over a distance of 50 meters. Jessica pushes with a Force of 30 Newtons over 100 meters.
a) How much Work does Jeff do?
3,000 J
b) How much Work does Jessica do?
3,000 J
c) Who does more work – Jeff or Jessica?
They do the same amount of work.
6. Julia is skateboarding along at a constant speed suddenly comes upon a rough part of the pavement. The
pavement exerts a constant frictional force of 45 N and Julia comes to a stop over a distance of 8 meters.
a) How much Work is done on Julia by the force of friction?
360 J
b) Did the energy of Julia change? If so, how?
Yes, Julia had a change in energy because work was done on her. The change in kinetic energy was
caused by the force of friction.
7. Julia is traveling along again on her skateboard at a constant speed holding a box that weighs 10 Newtons with
her hands as shown in the picture below. The force of gravity on the box is canceled out by the force of Julia’s
hand on the box, therefore the box does not move at all in the vertical direction. She does this over a distance of
50 meters.
a) How much Work is done by the force of Julia’s hands on the box? Assume
no air resistance.
0J
b) How does this compare to the change in energy of the box
There is no change in energy of the box because no work is done.
8. What is the power used when a 40 kg box is lifted 0.75 m in 2.2 s?
P = 134 W
9. How much force do I need to push a box 4.0 m in 3.0 s if I use 68 W of power?
F = 51 N [forwards]
10. I am pulling a 10.0 kg box with an applied force of 124N [35* above the horizontal] and I move the box 26.7
m. If the coefficient of friction is 0.14, what the work done on the box?
w = 2, 610.6 J
Potential Energy and Kinetic Energy Practice
Use your new knowledge about GPE and KE and the formulas for them to answer the following questions.
U = PE = mgh
KE = ½ mv2
v=
2·KE
m
where g = 9.8 m/s/s
11. What factors determine the gravitational potential energy that an object has?
The height, mass and the acceleration of gravity determine the potential energy that an object has.
12. Why is it more “spectacular” to drop a water balloon out of a 10th story window than a 2nd story window?
The water balloon that is dropped out of the 10th story window will hit the ground with a larger velocity
due to the fact that more potential energy was converted to kinetic energy. Since the balloon has a larger
velocity, it will cause a more “spectacular” splash of water when it hits the ground.
13. Why should you be more worried about a piano falling from the 2nd story than about a flowerpot falling off
the 2nd story?
.
More mass means that the object will have more potential energy. Since there is more potential energy at
the top, there will be more kinetic energy at the bottom. A larger kinetic energy means that the object will
hit you with a larger velocity, which will cause more damage to you.
14. What factors determine the kinetic energy that an object has?
The mass and velocity of an object determine the kinetic energy that an object has. If an object is in
motion, it has kinetic energy.
15. A 90 kg leopard sits in a 4 m high tree and waits for some prey to pass underneath.
a) What kind of energy does the leopard have?
Potential energy.
b) How much energy does the leopard have?
U = 3, 532 J
16. A 1500 kg Indy car races down a flat track at 100 m/s.
a) What kind of energy does the Indy car have?
Kinetic energy.
b) How much energy does the Indy car have?
K = 7, 500, 000 J
17. Jennifer is riding her bike at a constant speed down a flat road. She and the bike combined have a kinetic
energy of 625 Joules. Assuming the combined mass of Jennifer and the bike is 50 kg, how fast is she going?
V = 5.0 m/s [forwards]
Conservation of Energy
Total Energy at start (mechanical energy) = Total Energy at end (mechanical energy)
18. A toy rocket burns its fuel to launch itself into the air, where it rises to a height of 200 m and then
falls back to earth. Explain all the energy transformations that occur in this series of events. (Hint:
break it down step by step: first the launch, then the rise and then the fall)
Launch = only kinetic energy (no potential energy because there is no height)
Rise = Increasing potential energy, decreasing kinetic energy
Top = Only potential energy (no kinetic energy because the rocket has stopped moving before it
starts falling back down to Earth)
Fall = Increasing kinetic energy, decreasing potential energy
End = Only kinetic energy
19. A 200 kg gargoyle is perched on top of Notre Dame cathedral. He is 50 meters off the ground.
a) How much Gravitational Potential Energy (GPE) does the gargoyle have?
U = 98, 100 J
b) If it were to fall with no air resistance, how much kinetic energy would it have when it was just about
to hit the pavement underneath?
K = 98, 100 J
c) What will its speed be the moment before it hits the pavement?
v = 31.3 m/s
d) How much work has been done when the gargoyle hits the ground?
W = 98, 100 J
d) When it is only halfway down, what kinds of energy and and how much of each kind does it have?
It has both kinetic energy and potential energy. At this point they are equal, so:
U = K = 49, 050 J
20. A 60 kg gymnast on a trampoline leaves the trampline going 7 m/s.
a) How much kinetic energy does the gymnast have at the start?
K = 1, 470 J
b) How much gravitational potential energy will the gymnast have when they reach the highest point?
U = 1, 470 J
c) How high off the trampoline will they be at the highest point?
h = 2.5 m