Give an Example of a Situation in Which: (A) a Force Is Acting but There Is No Motion And

SPH 3U Review of Work, Power and Energy

1. Give an example of a situation in which: (a) a force is acting but there is no motion and therefore no work done. (b) there is motion, but since no force is acting to cause the motion, no work is done.

2. Describe a situation that explains the difference between work, power and energy.

3. Utility companies must take many factors into consideration when designing power plants that convert the gravitational potential energy of water into electricity. To generate the maximum amount of electricity at Niagara Falls, for example, all of the water should be diverted through turbines instead of letting some of it go over the falls. List several reasons why this is not done.

4. A 60 kg man and a 40 kg girl sit on identical swings. They are then each given a push so that in both cases the swings move, through the same angle from the vertical. How will their speeds compare as they swing through the bottom of the cycle? Explain your answer.

Use g = 9.80 N/kg. Assume applied forces act horizontally unless stated otherwise.

5. How much work must be done to lift a 20 kg sack of potatoes vertically 6.5 m?

6. If a small motor does 520 J of work to move a toy car 260 m, what force does it exert?

7. How long would it take a 500 W electric motor to do 1.50 x 105 J of work?

8. A boy fires a 60 g pebble with his slingshot. The pebble leaves the slingshot at 35 m/s. (a) How high above the slingshot will the pebble rise if it is fired straight up? (b) If the pebble is fired so that it goes in an arc and has a speed of 10 m/s at its maximum height, what will the maximum height be? (c) At what speed would an 80 g pebble have to be fired to reach the same height as the pebble in (a)? Assume that the 80 g pebble is also fired straight up.

9. A boy on a bicycle drags a wagon full of newspapers at 0.80 m/s for 30 min using a force of 40 N. How much work has the boy done?

10. It is estimated that one kilogram of body fat will provide 3.8 x 107 J of energy. A 60 kg mountain climber decides to climb a mountain 4000 m high. (a) How much work does the climber do against gravity in climbing to the top of the mountain? (b) If the body's efficiency in converting energy stored as fat to mechanical energy is 25%, determine the amount of fat the climber will use up in providing the energy required to work against gravity.

11. A chair lift takes skiers to the top of a mountain that is 300 m high. The average mass of a skier complete with equipment is 80 kg. The chair lift can deliver three skiers to the top of the mountain every 30 s. (a) Determine the power needed to do this task. (Assume skiers join the lift at full speed.) (b) If friction increases the power required by 25%, what power must the motors that are running the lift be able to deliver?

\sph3U\wpe\04fbd8d304e02d8212c5367db31c2d01.doc 12. A 50 kg bicyclist on a 10 kg bicycle speeds up from 5.0 m/s to 10 m/s. (a) What was the total kinetic energy before accelerating? (b) What was the total kinetic energy after accelerating? (c) How much work was done to increase the kinetic energy of the bicyclist? (d) Is it more work to speed up from 0 to 5.0 m/s than from 5.0 to 10.0 m/s?

13. A rodeo rider is riding a bucking bronco when he is thrown off. At the instant he leaves the horse he is located 1.6 m above the ground and is moving straight up at 4.0 m/s. (a) What maximum height above the ground does the rider reach? (b) At what speed will the rider hit the ground?

14. What is / are the energy transfers for a roller coaster cart starting from rest at the top of a hill, rocketing downward and then transitioning to a stop at the end of the track?

15. A car’s efficiency is not 100%. Draw an energy transfer diagram using this information: Heat from engine 70% Heat in road 4% Sound to air 5% What is the efficiency of the car?

16. A small glowing hot piece of magnesium ribbon has a temperature of about 1200°C and a very large block of aluminum has a temperature of only about 20°C. Which has the greatest amount of thermal energy? Explain.

17. How much heat is required to raise the temperature of 2.0 kg of water from 25°C to 80°C?

18. How much heat is lost when 1.00 kg of copper cools from 90°C to 20°C?

19. When 2.1 x 103 J of heat is added to 100 g of a substance, its temperature increases from 15°C to 40°C. What is the specific heat capacity of the substance?

20. How are the principle of heat exchange and the conservation of energy similar concepts?

21. A typical hairdryer has a power rating of 1600 W. If you run it for 12 minutes drying your hair, and the price of electricity is 5.5¢ / kW·h, how much will it cost to dry your hair for 5 days?

F 22. Does a ramp help reduce the amount of work you do to raise a barrel of water to a height of 10.0 m? Explain. 80. kg 16 m 64 m 23. Find the energy lost to frictional forces in the F = 30 N diagram. Find F. f

24. Explain why it is impossible to have a natural system that is 100% efficient.

Answers: 4. 1274 J b) 57.4 m b) 29 400 W 5. 2 N 2.352 MJ c) 35 m/s 12. a) 750 6. 5 min b) 0.21 kg J 7. 1.32 x 106 J 9. 57.6 kJ 11. a) 23 b) 3000 J 8. a) 62.5 m 10. a) 520 W c) 2250 J \sph3U\wpe\04fbd8d304e02d8212c5367db31c2d01.doc d) No 13. a) 2.4 15. 21% 19. 840 J/(kg·C) 5 m 17. 4.62 x 10 J 21. ¢8.8 b) 6.9 m/s 18. 27.3 kJ 23. 1920 J; 226 N

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