# WORK AND POWER solutions Work is done when

- Slides: 28

WORK AND POWER solutions

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

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

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

4. How much work is done in holding a 15 N sack of potatoes 2 m off the ground while waiting in line at the grocery store for 3 minutes? Work = Force x Distance W=0 J

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=Fxd 2. 7 = 4. 5 x d d = 2. 7/4. 5 = 0. 6 m

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=Fxd 405 = F x 15 F = 405 / 15 F = 27 N

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 = 237. 825 k. J

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=Fxd 3650 = 2470 x d d = 3650/2470 d = 1. 48 m

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=Fxd 16700 = F x 3. 05 F = 16700/3. 05 F = 5475 N

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=Fxd W = 2000 x 150 W = 300000 J = 300 k. J

11. If a small motor does 520. J of work to move a toy car 260. m, what force does it exert? W=Fxd 520 = F x 260 F=2 N

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 = 75000 J = 75 k. J b. Wnet = (300 – 300) x 750 Wnet = 0 J

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; 0. 454 kg = 1 lb) If the wall does not move, no work is done because d = 0 m.

14. 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=vxt d = 0. 8 m/s x 30 minutes x 60 seconds/minute d = 1440 m W=Fxd W = 40 x 1440 W = 57600 J = 57. 6 k. J

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

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.

17. The SI unit for Power is the Watt(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 = 190000 x 9 W = 1710000 J = 1710 k. J

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 = 250000/7. 5 P = 33333 W = 33. 3 k. W

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

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

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

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

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=Fxd 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 equal forces friction force equals apply force = 5 N, so friction force = 5 N

25. 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=Fxd/t P = (30 x 9. 8) x 10 / 5 P = 588 W

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