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WORK AND POWER solutions
1. How much work is done in holding a 15 N sack of
potatoes 2m off the ground while waiting in line at the
grocery store for 3 minutes?
Work = Force x Distance
W = 0 J
2. You must exert a force of 4.5 N on a book to slide it
across a table. If you do 2.7 J of work in the process, how
far have you moved the book?
W = F x d
2.7 = 4.5 x d
d = 2.7/4.5 = 0.6 m
3. A child pulls a sled up a snow-covered hill. The child
does 405 J of work on the sled. If the child walks 15 m up
the hill, how large of a force must the child exert?
W = F x d
405 = F x 15
F = 405 / 15
F = 27 N
4. How much work is done on a small car if a 3150 N force
is exerted to move it 75.5 m to the side of the road?
W = 3150 x 75.5
W = 237.825 kJ
5. A crate is being lifted into a truck. If it is moved with a
2470 N force and 3650 J of work is done, then how far is
the crate being lifted?
W = F x d
3650 = 2470 x d
d = 3650/2470
d = 1.48 m
6. If 16,700 J of work is done to shoot the human
cannonball down a 3.05 m barrel, then how much force is
applied to the person to fire them out the cannon?
W = F x d
16700 = F x 3.05
F = 16700/3.05
F = 5475 N
7. An elephant pushes with 2000 N on a load of trees. It
then pushes these trees for 150 m. How much work did the
elephant do?
W = F x d
W = 2000 x 150
W = 300000 J = 300 kJ
8. If a small motor does 520. J of work to move a toy car
260. m, what force does it exert?
W = F x d
520 = F x 260
F = 2 N
9. A girl pushes her little brother on his sled with a force of
300. N for 750. m. How much work is this if the force of
friction acting on the sled is (a) 200. N, (b) 300. N?
a. Wnet = Fnet x d
Wnet = (300 – 200) x 750
Wnet = 100 x 750
Wnet = 75000 J = 75kJ
b. Wnet = (300 – 300) x 750
Wnet = 0 J
10. A 75.0 kg man pushes on a 5.0 x 105 ton wall for 250 s
but it does not move. How much work does he do on the
wall? (2000 lb = 1 ton; 0.454 kg = 1 lb)
If the wall does not move, no work is done because
d = 0 m.
11. A boy on a bicycle drags a wagon full of newspapers at
0.800 m/s for 30.0 min using a force of 40.0 N. How much
work has the boy done?
d = v x t
d = 0.8 m/s x 30 minutes x 60 seconds/minute
d = 1440 m
W = F x d
W = 40 x 1440
W = 57600 J = 57.6 kJ
12. Power is the rate at which work is done.
13. When calculating power, you should use the formula P
= work divided by time. In this formula, “P” stands for
power, W stands for work, and t for time.
14. The SI unit for Power is the Watt(s).
15. An 190,000 W engine can accelerate from rest to a top
speed in 9 s. How much work did the engine do?
P = W/t
W = Pt
W = 190000 x 9
W = 1710000 J = 1710 kJ
16. Another engine reaches its top speed from rest in 7.5
s. It is able to perform 250,000 J of work in that time. How
much power does this engine have in that time?
P = W/t
P = 250000/7.5
P = 33333 W = 33.3kW
17. If a runner exerts 350 J of work to make 125 W of
power, then how long did it take the runner to do the work?
P = W/t
t = W/P
t = 350/125
t = 2.8 seconds
18. A person weighing 600 N gets on an elevator. The
elevator lifts the person 6 m in 10 seconds. How much
power was used?
P = W/t
P = Fd/t
P = 600 x 6 / 10
P = 360 W
19. How much time is needed to produce 720 Joules of
work if 90 watts of power is used?
P = W/t
t = W/P
t = 720/90
t = 8 seconds
20. A set of pulleys lifts an 800 N crate 4 meters in 7
seconds. What power was used?
P = W/t
P = Fd/t
P = 800 x 4 / 7
P = 457 W
21. A force of 5.0 N moves a 6.0 kg object along a rough floor at a constant speed of 2.5 m/s.
a. How much work is done in 25 s?
W = F x d
W = 5 x (2.5 x 25)
W = 312.5 J
b. What power is being used?
P = W/t
P = 312.5/25
P = 12.5 W
c. What force of friction is acting on the object?
constant velocity dynamic equilibrium
dynamic equilibrium equal forces
equal forces friction force equals apply force
apply force = 5 N, so friction force = 5 N
22. A car uses 2,500 Joules in 25 seconds. Find
power.
P = W/t
P = 2500/25
P = 100 W
26. How much power does it take to lift 30.0 N 10.0 m
high in 5.00 s?
P = W/t
P = F x d/t
P = 30 x 10 / 5
P = 60 W
27. How much power does it take to lift 30.0 kg 10.0 m
high in 5.00 s?
P = W/t
P = F x d / t
P = (30 x 9.8) x 10 / 5
P = 588 W