Assignment 1 Unit 3 Work, Power, Efficiency, and Potential Energy Name: Multiple Choice. Show workings where necessary. 1. In which situation is work not done? A) a frozen turkey is carried upstairs B) a frozen turkey is carried on level ground C) a frozen turkey is dropped D) a frozn turkey is carried dowstairs 2. What is 1 joule (J) is equivalent to? A) kg m/s C) kg m 2 /s 2 B) watt D) kg m 3 /s 2 3. In which case is negative work done? A) The work done by Earth on a ball as the ball, which was initially at rest, falls 3.0 m down towards Earth. B) A eastward force is applied to an eastward moving soccer ball that is already moving at a constant velocity to increase its speed in that direction. C) A cart is moving at a constant velocity of 10 m/s [W] when a 0.5 N [downward] force is applied. D) The work done by Earth on an arrow as it is fired 200 m straight up into the air. 4. Mary exerts a certain force to lift a dumbbell 3.0 m straight above her head in 20 s. If she wanted to complete the same task in 10 s, how much work would have to do? A) twice as great B) the same C) four times as great D) half as great 5. A 100-kg person has an average power output of 140 W while walking up a 0.50-km high mountain. How long does it take the person to climb the mountain? (Show workings) A) 9.7 10 1 h C) 5.8 10 1 h B) 7.1 10 1 h D) 3.6 10 2 h 6. An elevator motor lifts a load of 2500 kg a height of 12.0 m in 3.00 seconds. The energy required by the motor to accomplish this task is 300 000 J. What is the efficiency of the elevator? (Show workings) A) 10.0% B) 30.0% C) 98.0% D) 102% 7. A force of 25.0 N is required to raise a 500-N load with a pulley system. When the rope is pulled 10.0 m, the load is raised 0.200 m. What is the efficiency of this pulley? (Show workings) A) 60.0% B) 50.0% C) 5.00% D) 40.0% 8. A cheerleader is thrown vertically upward into the air. As the cheerleader rises, which one of the following quantities increases? A) gravitational potential energy C) kinetic energy B) speed D) acceleration
9. In the following figure, a 0.60-kg ball is about to fall to the step below. What is the gravitational potential energy of the ball relative to the step? (Show workings) A) 2.9 10 1 J C) 5.9 10 1 J B) 8.8 10 1 J D) 6.0 10 0 J 10. What work is required to compress a spring with a force constant of 290.0 N/m a total of 12.3 mm? (Show workings) A) 0.0219 J B) 3.57 J C) 1.78 J D) 23.6 J 11. Paul applies a 320.0 N force 34.0 to the horizontal to push a go-cart a distance of 43.0 m, as shown below. How much work does Paul do on the go-cart if we ignore friction? {2} 12. A muscle-bound contestant in a popular game show wins a competition by lifting a 30.0 kg block of concrete 1.4 m above the ground and holding it there for 95 seconds. The announcer for the game states that the contestant did no work on the block while he was holding it. Is he correct? Explain. {2} 13. A car's engine applies the force needed to overcome friction while the car moves a distance of 13.0 km at constant speed. The mass of the car is 1650 kg and the coefficient of friction is 0.245. Calculate the amount of work done. {4} 14. A 1500 kg car slows down from 25 m/s to 12 m/s in 6.5 s. Calculate the work done on the train. {8}
15. Calculate the work by the object in the graph below. {5} 16. An elevator motor provides 45.0 kw of power while lifting a 2000-kg elevator 35.0 m. If it takes the elevator 20.0 s to accomplish this task, determine the mass of the elevator. {4} 17. The winch on a pickup truck is used to tow a snowmobile that has become stuck. The sled has a mass of 305.0 kg and the coefficient of friction between the track and the snow is 0.195. The sled is moved a distance of 7.0 m in a time of 8.0 s at a constant speed. Calculate the power developed by the winch. 18. a) A 15.00 hp snowmobile (note that 1 hp = 746 W. ) requires a force of 350.0 N to move a sled load of wood along a frozen marsh. What distance will the snowmobile have gone in 0.5 minutes in order to tow the wood? {4} b) Can a less powerful snowmobile be used to tow a sled of wood that is identical to the one in the previous question? Use your understanding of the concept power to explain how the situation would be different if a less powerful machine was used. {2}
19. A 5000 W winch has been employed to lift an sailor from a stranded ship. A rescue worker with a mass of 110 kg goes down the cable to assist the man with a mass of 85 kg. If the chopper is hovering 125 m above the boat how long will it take to get the two men on board once the lift is started? 20. Explain why it is easier to push a piano up a ramp, but it is more efficient to lift it vertically to that height. {4} 21. A piano with a mass of 400 kg is loaded onto a delivery truck by rolling it 3.0 m along an incline. The force needed to move the piano along the incline is 400 N. If the incline is 55 % efficient, how high does the piano have to be raised for it to be placed in the truck? 22. A large scale hydro electric project requires the use of 7 meter high concrete blocks to form a dam. A project engineer estimates that a net force of 3200.0 N is required to move each block a distance of 8.0 m away from an unstable area of the site. To do this 2 choices for a pulley system can be used. With pulley system A, a single worker can do the job by using a net force of 1550.0 N to haul in 22.0 m of rope. With pulley system B, a single worker can do the job by using a net force of 1400.0 N to haul in 30.0 m of rope. a) Use the given information to calculate W out for each pulley system. {2} b) Use this information to calculate W in for each pulley system. {4} c) Use parts (a) and (b) to help you to calculate the efficiency of each pulley system. {4}
23. The 8.74 kg box below was pushed up a ramp. What is the gravitational potential energy of the box relative to the floor? {4} 1 1. 7 m 8.74 kg 29.0 24. Devon pushes a 58.74 kg box up a ramp using a force of 275 N. He walks on the ramp, pushing the box for 15.6 m. What is the height of the ramp and what angle did it make with the ground? Ignore friction. 25. A spring with a spring constant of 340 N/m has a 1.2 kg mass hanging from it. What is the extension of the spring and how much potential energy does it have once the mass is suspended? {5} 26. A bungee cord needs to transfer 2.0 10 6 J of energy. A 10-kg mass extends the bungee cord 1.3 m. What is the maximum extension of the bungee cord? {5} 27. A 1600 g wooden block is hung from a spring mounted on a ceiling and this stretches the spring a distance of 6.0 cm. (a) Determine the spring constant for this spring. (b) How much more elastic potential energy would the spring gain if an average force, F, stretches the spring a further distance of 10.0 cm?