Unit 4 Work, Power, and Energy Practice Test
1) What conditions must exist in order for a force to do work on an object?
2) What formula relates work and power?
3) How much work is done when a vertical force acts on an object moving
horizontally?
4) A desk exerts an upward force to support a computer resting on it. Does
this force do work? Explain.
5) Two cars have the same weight, but one of the cars has an engine that
provides twice the power of the other. Which car can make it to the top
of the mountain pass first? Which car does more work to reach the pass?
6) You carry two heavy bags of groceries upstairs to your kitchen. Will you do
more work on the bags if you carry them up one at a time? Explain.
7) How much work does a 25-newton force do to lift a potted plant from the
floor to a shelf 1.5 meters high?
8) You lift a large bag of flour from the floor to a 1-meter high counter, doing
100 joules of work in 2 seconds. How much power do you use to lift the
bag of flour?
9) Your family is moving to a new apartment. While lifting a box 1.5 meters
straight up to put it on a truck, you exert an upward force of 200 newtons
for 1 second. How much power is required to do this?
10) You lift a book from the floor to a bookshelf 1 meter above the ground.
How much power is used if the upward force is 15 newtons and you do
the work in 2 seconds?
11) You apply a horizontal force of 10 newtons to pull a wheeled suitcase at
a constant speed of 0.5 m/s across the ground. How much power is used?
(Hint: The suitcase moves 0.5 m/s. Consider how much work the force
does each second and how work is related to power.)
12) Describe the relationship between work and energy.
13) How is the kinetic energy of an object determined?
14) What factors determine the gravitational potential energy of an object?
15) Give an example of each of the major forms of energy?
16) When you heat a pot of water over a flame, what form of energy is
added to the water?
17) What kind of energy is represented by an archer stretching a bow string?
18) Can an object have both kinetic energy and potential energy at the
same time? Explain.
19) Describe how energy can be converted from one form to another in a
wind-up toy.
20) What does the law of conservation of energy state?
21) As an object falls in free fall, what energy change is taking place?
22) Describe the energy of a playground swing at its highest position.
23) Why does a bouncing ball rise to a lower height with each bounce?
What energy conversion is taking place?
24) To begin a dive, a diver jumps into the air and then lands on the diving
board, causing it to bend. What type of energy conversions occur as the
board springs back and propels the diver into the air?
25) A 0.15 kg ball is through into the air and rises to a height of 20 meters.
How much kinetic energy did the ball initially have?
26) A 125 gram steel ball with a kinetic energy of 0.25 Joules rolls along a
horizontal track. How high up an inclined track will the ball roll if friction
can be ignored?
27) If you throw a baseball straight up with a speed of 5 m/s, what will be the
speed of the ball when it comes back to your hand? (Ignore air
resistance).
28) A 1000 kg car is coasting at 10 m/s toward a hill that is 10 meters high. Will
the car make it to the top of the hill if the driver does not step on the gas
pedal? Explain.
Answers
1) Some of the force must act in the same direction as the object moves.
2) Power = Work ÷ Time
3) No work is done.
4) The supporting force does no work because the object it acts on (the
computer) does not move.
5) The car with the more powerful engine will reach the top first. Both cars do
the same amount of work to reach the top.
6) Carrying one bag at a time uses only half the force, but required that the
force be applied through twice the distance. The work done is the same
in both cases.
7) Work = 38 Joules
8) Power = 50 Watts
9) Power = 300 Watts
10) Power = 7.5 Watts
11) Power = 5 Watts
12) Energy is the ability to do work, and work is the transfer of energy.
13) By multiplying half an object’s mass by the square of its speed.
14) Mass, acceleration due to gravity, and height relative to a reference
level.
15) Mechanical: a bouncing ball
Thermal: molten steel or fire
Chemical: gasoline
Electrical: lightning
Electromagnetic: light
Nuclear: nuclear power plants
16) Thermal Energy
17) Elastic Potential Energy
18) Yes. Kinetic energy and potential energy are not mutually exclusive. A
falling object has both kinetic and potential energy as it falls.
19) Elastic potential energy stored in the wound-up spring is converted to
kinetic energy as the spring unwinds and the toy moves.
20) Energy cannot be created nor destroyed.
21) Gravitational potential energy is converted into kinetic energy as the
object falls.
22) The swing has maximum gravitational potential energy and no kinetic
energy at its highest position.
23) Friction within the ball, between the ball and the ground, and between
the ball and the air converts kinetic energy into thermal energy. Each
bounce has less kinetic energy.
24) The elastic potential energy of the board is converted into both kinetic
energy and gravitational potential energy (because the diver’s height
increases while the diver is still in contact with the board).
25) KE = 29 Joules
26) h = 0.20 meters
27) v = 5 m/s
28) At a height of 10 meters, PE would be about 100,000 Joules. Initial KE =
50,000 Joules. The car would only reach a height of 5 meters, ignoring
friction.