Energy Unit Concept Review Questions
Conceptual Physics
Name:
Period:
Work and Energy Questions 1- 7 Read pages 103-104 in your book
1) How much Work does a dog do while pulling with 50 N on a sled for 200 m?
W  Fd  50 N (200m)  10000 J
Nm  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?
It is the same since the dog and the sled are attached they are considered a system. So what energy one part of the system has
the other has as well.
3) How is Work related to Force? Hint: look at the equation from class notes.
It is a direct relationship. The more force put in the more work is done. The less force put in the less work is done.
4) What is energy? Why do we care about it? Give two examples from every day life.
Energy is the ability to do work. We care because nothing can exert a force without energy.
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?
W  Fd  60 N (50m)  3000 J
b) How much Work does Jessica do?
W  Fd  30 N (100m)  3000 J
c) Who does more work – Jeff or Jessica? Same
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? Pay close attention to the sign of your answer.
W  Fd  45 N (8m)  360 J
b) Did the energy of Julia change? If so, how?
Yes she lost 360J of energy. Because when she was stopped all her KE was gone given over the the heat and sound
from the rough pavement.
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.
None: her hands are exerting a force up but they are not actually making the box move up
so no work is done on the box.
W  10 N (0m)  0
b) How does this compare to the change in energy of the box?
It is the same, the box is not feeling a net force or accelerating so it does not gain or lose
energy.
GPE and KE Practice read pages 105-108
Use your new knowledge about GPE and KE and the formulas for them to answer the following questions.
KE = ½ ms2
GPE = mgh
h=
GPE
mg
s=
2·KE
m
where g = 9.8 m/s/s
8. What factors determine the gravitational potential energy that an object has?
Mass
Gravity
Height
9. Why is it more spectacular to drop a water balloon out of a 10th story window than a 2nd story window?
It has more energy because of the greater height and can do more work when it hits the ground.
10. Why should you be more worried about a piano falling from the 2nd story than about a flowerpot falling off
the 2nd story?
Because the piano has more energy because of its mass. And it can do a lot more work when it reaches the
ground.
11. What factors determine the kinetic energy that an object has?
Mass
Speed
12. Why is it more dangerous to get hit by a semi-truck than by a VW bug going the same speed?
Because the semi-truck has more mass, giving it more energy, allowing it to do more work on anything it
hits.
13. Why are your bumpers able to protect you from a 5 mph crash but not from a 30 mph crash?
Because you energy is directly dependent on speed. And the faster you are going the more energy you
have the more work is done in the collision.
14. 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?
GPE
b) How much energy does the leopard have?
GPE  mgh  90kg(9.8 ssm )( 4m)  3528 J
15. A 1500 kg Indy car races down a flat track at 100 m/s.
a) What kind of energy does the Indy car have?
KE
b) How much energy does the Indy car have?
KE 
1
2
ms 2  1500kg(100 ) 2  7.5 x10 6 J
1
m
2
ss
16. 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?
s
2 KE

m
2(625 J )
 5 ms
50kg
17. It takes 4184 J of energy to equal 1 Food calorie. To use 1 Food calorie climbing stairs you need to give
yourself 4184 J of gravitational potential energy. How high will YOU have to climb to give yourself that much
gravitational potential energy? (Hint: to find your mass in kilograms, take your weight in pounds and divide by 2.2. This
works because there is a direct relationship between mass and weight even though they measure different things. )
Conservation of Energy read pages 109-111
Total Energy at start = Total Energy at end
GPE = mgh
h=
GPE
mg
KE = ½ ms2
s=
2·KE
m
where g = 9.8 m/s/s
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)
GPE
KE
becoming
GPE
KE
Chemical
potential
energy
Heat,
Sound &
Light
GPE
becoming
KE
KE
Heat
and
Sound
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?
GPE  mgh  200kg(9.8 ssm )(50m)  98000 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?
98000J the amount of GPE it had before it fell would be converted to KE by the time it hit.
c) What will its speed be the moment before it hits the pavement?
s
2 KE 2(98000 J )
 31.3 ms
M
200kg
d) When it is only halfway down, what kinds of energy and and how much of each kind does it have?
Halfway down its’ total energy of 98000J would be split evenly between GPE and KE.
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?
KE 
1 2 1
ms  60kg(7 ms ) 2  1470 J
2
2
b) How much gravitational potential energy will the gymnast have when they reach the highest point?
The same amount all the KE at the beginning has become GPE at the top.
c) How high off the trampoline will they be at the highest point?
GPE
1470 J
h

 2.5m
mg
60kg(9.8 ssm )