1. Work and Power
solutions

2. Work is done when an object moves through a distance because of a Force acting upon the object.

3. 2. When calculating work, you should use the formula: work = force X distance.

4. 3. The SI unit for work is the joules
3. The SI unit for work is the joules. It is represented by the letter __J____.

5. 4. 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

6. 5. You must exert a force of 4
5. 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

7. 6. A child pulls a sled up a snow-covered hill
6. 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

8. 7. 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 = KJ

9. 8. A crate is being lifted into a truck
8. 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

10. 9. If 16,700 J of work is done to shoot the human cannonball down a 3
9. 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

11. 10. An elephant pushes with 2000 N on a load of trees
10. 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 = J = 300 kJ

12. 11. If a small motor does 520. J of work to move a toy car 260
11. 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

13. 12. A girl pushes her little brother on his sled with a force of 300
12. 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 = J = 75kJ
b. Wnet = (300 – 300) x 750
Wnet = 0 J

14. 13. 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; kg = 1 lb)
If the wall does not move, no work is done because d = 0 m.

15. 14. A boy on a bicycle drags a wagon full of newspapers at 0
14. A boy on a bicycle drags a wagon full of newspapers at 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 = J = 57.6 kJ

16. 15. Power is the rate at which work is done.

17. 16. 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.

18. 17. The SI unit for Power is the Watt(s).

19. 18. An 190,000 W engine can accelerate from rest to a top speed in 9 s
18. 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 = x 9
W = J = 1710 kJ

20. 19. Another engine reaches its top speed from rest in 7. 5 s
19. 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 = /7.5
P = W = 33.3kW

21. 20. 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

22. 21. A person weighing 600 N gets on an elevator
21. 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

23. 22. 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

24. 23. A set of pulleys lifts an 800 N crate 4 meters in 7 seconds
23. 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

25. 24. 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 = 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

26. 25. A car uses 2,500 Joules in 25 seconds. Find power
25. A car uses 2,500 Joules in 25 seconds. Find power. P = W/t P = 2500/25 P = 100 W

27. 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

28. 27. How much power does it take to lift 30. 0 kg 10. 0 m high in 5
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