Physics
Work-Energy-Power Review Problems
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1. In physics, work is defined as
2. If you lift two loads up one story, how much work do you do compared to lifting just one load up
one story?
3. If you lift one load up two stories, how much work do you do compared to lifting one load up
only one story?
4. If Nellie Newton pushes an object with twice the force for twice the distance, she does how
much more work?
5. The unit of work is the
6. Power is defined as
7. The unit of power is the
8. Potential energy is _____________ energy.
9. Doubling the height of an object causes its PE to increase by ________ times.
10. Doubling the velocity of an object causes its KE to increase by ________ times.
11. How much farther will a car traveling at 100 km/s skid than the same car traveling at 50 km/s?
12. An arrow in a bow has 70 J of potential energy. Assuming no loss of energy to heat, how much
kinetic energy will it have after it has been shot?
13. Where is it stated that energy can be changed from one form to another, but the total amount
must remain constant.
14. As a pendulum rises into the air, __________ energy is converted to __________ energy.
15. When a car’s speed triples, its KE increases by ___________ times.
17. A woman can lift barrels a vertical distance of 1 meter or can roll them up a 2-meter long ramp
to the same elevation. If she uses the ramp, the applied force required is
18. A job is done slowly, and an identical job is done quickly. Both jobs require the same amount of
work but different amounts of __________.
19. Which requires more work: lifting a 70-kg sack vertically 2 meters or lifting a 35-kg sack
vertically 4 meters?
20. A ball is thrown into the air with 100 J of kinetic energy, which is transformed to gravitational
potential energy at the top of its trajectory. When it returns to its original level after encountering
air resistance, its kinetic energy is
21. An object that has kinetic energy must be
23. Which has greater kinetic energy, a car traveling at 30 km/h or a half-as-massive car traveling at
60 km/h?
24. A person on a roof throws one ball downward and an identical ball upward at the same speed.
The ball thrown downward hits the ground with 140 J of kinetic energy. Ignoring air friction,
with how much kinetic energy does the second ball hit the ground?
25. An object at rest may have
26. A heavy object and a light object are released from rest at the same height and time in a vacuum.
As they fall, they have equal
28. If the velocity of a moving object doubles, then what else doubles?
29. All simple machines ideally work on the principle that
30. The ratio of output force to input force of a simple machine is called the
____ 31. A pulley system can
____ 32. The ratio of useful work output to total work input is called the
____ 33. About how efficient is a typical gasoline-burning car engine? ____
34. A small economy car (low mass) and a limousine (high mass) are pushed from rest across a
parking lot, equal distances with equal forces. The car that receives more kinetic energy is the
____ 35. A popular toy consists of an aligned row of identical elastic balls suspended by strings so they
touch each other. When two balls on one end are elevated and released, they slam into the array
of balls and two balls pop out the other side. If instead, one ball popped out with twice the speed,
this would be a violation of the conservation of
____ 36. Consider molecules of hydrogen gas and molecules of heavier oxygen gas that have the same
kinetic energy. The molecules with the greater speed are
____ 37. How many joules of work are done on a box when a force of 25 N pushes it 3 m?
____ 38. How much power is required to do 40 J of work on an object in 5 seconds?
____ 39. How much work is done on a 60-N box of books that you carry horizontally across a 6-m room?
____ 40. How much work is done on a 20-N crate that you lift 2 m?
____ 41. How much power is expended if you lift a 60 N crate 10 meters in 1 second?
____ 42. Suppose a moving car has 3000 J of kinetic energy. If the car's speed doubles, how much kinetic
energy will it then have?
____ 43. It takes 80 J to push a large box 8 m across a floor. Assuming the push is in the same direction as
the move, what is the magnitude of the force on the box?
____ 44. If Kelly the skater's speed increases so he has three times the momentum, then his kinetic energy
increases by
____ 45. A 60-N object moves at 1 m/s. Its kinetic energy is
____ 46. Sue can easily lift a 45.0-N rock with the help of a lever. When she pushes down with 20.0 N of
force, she lifts the rock 0.3 meters. How far does she move her arms to do this?
____ 47. A pulley has two supporting strands. In order for it to lift a load 1 meter, the person pulling will
have to pull a distance of
____ 48. A frictionless inclined plane is 8.0 m long and rests on a wall that is 2.0 m high. How much force
is needed to push a block of ice weighing 300.0 N up the plane?
____ 49. A certain jack has a theoretical mechanical advantage of 300. However, due to frictional forces,
the actual mechanical advantage is only 100. What is the efficiency of the jack?
____ 50. A machine puts out 100 watts of power for every 1000 watts put into it. The efficiency of the
machine is
True/False
Indicate whether the sentence or statement is true or false.
____ 51. In physics, the amount of work done on an object is the product of the force exerted on the object
times the time the object moves.
____ 52. The unit of work is called the joule.
____ 53. The rate at which work is done is called energy.
____ 54. The unit of power is the watt.
____ 55. The rate at which work is done is called power.
____ 56. The energy an object has by virtue of its location is its potential energy.
____ 57. The energy an object has by virtue of its motion is its kinetic energy.
____ 60. When we carry an object across a room, without lifting it or setting it down, we do no physical
work on it.
____ 61. More power is needed to carry a heavy suitcase slowly up a flight of stairs than to carry the
suitcase quickly up the same flight of stairs.
____ 62. Energy transforms from one form to another with no net loss or gain.
____ 63. A pulley system can change the force required to lift an object, however it cannot affect the
length of rope pulled while lifting the object.
____ 64. Using an inclined plane is an effective way to lift a heavy object that you otherwise couldn't lift.
Problem
68. What is the work done in lifting 60 kg of blocks to a height of 20 m?
69. What is the work done in raising a 20-kg block 5 m vertically?
70. What amount of work is done on a chair that is pushed 9 m across a floor by a horizontal 30-N
net force?
71. If possible, determine the power expended when a barbell is raised 5.0 m in 2 s.
72. A toy cart moves with a kinetic energy of 10 J. If its speed is doubled, what will its kinetic
energy be?
73. In raising a 3000-N piano with a pulley system, it is noted that for every 2 m of rope pulled
down, the piano rises 0.2 m. Ideally, what force is needed to lift the piano?
74. A car traveling at 50 km/h will skid 20 m when its brakes are locked. If the same car is traveling
at 150 km/h, what will be its skidding distance?
75. What amount of work can a 600-W motor do in 4 minutes?
76. A couch potato might consume 7 million J of energy in a day. What is this rate of energy
consumption in watts?
77. A 30-kg girl runs up the staircase to a floor 5 m higher in 8 seconds. What is her power output?
78. At what height does a 100-kg mass have a potential energy of 20,000 J relative to the ground?
79. The 4.0-kg head of an ax is moving at 4.0 m/s when it strikes a log and penetrates 0.01 m into the
log. What is the average force the blade exerts on the log?