1. Work, Power Problems Answers
1. (a) A 5.0-kg box is pushed across the floor at constant velocity by
a force of 20 N. How much work is done to push the box 7.5 m?
W = F d
= ( 20 N )( 7.5 m )
W = 150 N m
W = 150 joules
W = 150 J

2. 1. (b) The box is then lifted 3.0 m. How much work is done?
W = F d
Force needed to lift box = weight of box
= m g
= ( 5.0 kg )( 9.8 m/s2 )
= 49 N
W = F d = ( 49 N )( 3.0 m )
W = 147 J

3. 2. The box in #1 is carried horizontally and placed on a shelf 3.0 m
high. How much work is done to carry the box horizontally?
W = 0
Displacement of object must be in the same direction
as the force in order for work to be done
When object is carried, displacement is horizontal, while
the exerted force is vertical (against gravity)

4. 3. A sled is dragged by a rope with a tension of 64 N at an angle of 25o
with the horizontal. Find the work done in dragging the sled 42.0 m.
64 N
Displacement is horizontal,
so need to find horizontal
component of force
25o Fh
42.0 m Fh
64 N
( 64 N )
cos 25 =
64 N
Fh = N

5. 3. A sled is dragged by a rope with a tension of 64 N at an angle of 25o
with the horizontal. Find the work done in dragging the sled 42.0 m.
64 N
25o
58.0 N
42.0 m
Then W = Fh d
= ( 58.0 N )( 42.0 m )
W = J = J

6. 4. Bob lifts 120 kg to a height of 2. 5 m in 1. 8 s
4. Bob lifts 120 kg to a height of 2.5 m in 1.8 s. Tom can lift the mass to
the same height in 1.4 s.
(a) Which man did more work?
They each did the same amount of work
Both Bob and Tom exerted a force in a vertical direction (against gravity) and moved the box the same vertical distance

7. 4. Bob lifts 120 kg to a height of 2. 5 m in 1. 8 s
4. Bob lifts 120 kg to a height of 2.5 m in 1.8 s. Tom can lift the mass to
the same height in 1.4 s.
(b) Which man exerted more power?
Tom lifted the mass in a shorter amount of time, so
Tom exerted more power.

8. 4. Bob lifts 120 kg to a height of 2. 5 m in 1. 8 s
4. Bob lifts 120 kg to a height of 2.5 m in 1.8 s. Tom can lift the mass to
the same height in 1.4 s.
(c) Calculate the power exerted by each man. W
W = F d
P = t
W = F d
= Wt. d
= m g d
= ( 120 kg )( 9.8 m/s2 )( 2.5 m )
= J

9. 4. Bob lifts 120 kg to a height of 2.5 m in 1.8 s. Tom can lift
the mass to the same height in 1.4 s.
(c) Calculate the power exerted by each man.
W = J W
2940 J
Bob:
P = =
= J/s t
1.8 s
= watts
= W W
2940 J
Tom:
P = =
= J/s t
1.4 s
= watts
= W