Physics Study Guide Work and Energy

Solve the following problems.

1. A rope is used to pull a metal box 15 m across the floor. The rope is held at an angle of 46o with the floor and a force of 628 N is used. How much work does the force on the rope do? 2. A jukebox that weighs 575 N is lifted a distance of 20 m straight up by a rope. The job is done in 10 s. What power is developed in watts? 3. A rock climber wears a 7.5 kg knapsack while scaling a cliff. After 30 min, the climber is 8.2 above the starting point. a. How much work does the climber do on the knapsack? b. If the climber weights 645 N, how much work does she do lifting herself and the knapsack? c. What is the average power developed by the climber? 4. An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 m in 35 s. How much force does the motor exert? 5. A rock is used to drive a wedge into a log to split it. When the wedge is driven 20 cm into the log, the log is separated a distance of 5 cm. A force of 1.9 X 104 N is needed to split the log, and the sledge exerts a force of 9.8 X 103 N. a. What is the IMA of the wedge? b. Find the MA of the wedge. c. Calculate the efficiency of the wedge as a machine. 6. Bob uses a pulley system to raise a 225 N crate 16.5 m. A force of 129 N is exerted and the rope is pulled 33 m. a. What is the mechanical advantage of the pulley system? b. What is the efficiency of the system? 7. Jane exerts a force of 225 N on a lever to raise a 1.25 X 103 N rock a distance of 0.13 m. If the efficiency of the lever is 88.7%, how far did the boy move his end of the lever? 8. Find the horizontal Kinetic Energy and vertical Potential Energy of a bullet with a mass of 4.2g and a velocity of 965 m/s if the rifle is held 2.5 m above the ground. 9. A 50 kg shell is shot from a cannon at Earth’s surface to a height of 4.0 X 102 m. a. What is the potential energy of the shell at its maximum height? b. What is the change in potential energy of the system when the shell falls to a height of 2.0 X 102 m? 10. Superman, with a mass of 225 kg, was flying around one day looking out for trouble. He was flying at a height of 500 m. Suddenly, a piece of Kryptonite rope came out of nowhere and wrapped around Superman. The Kryptonite rope took Superman’s powers and he fell to the ground. a. What was Superman’s Potential Energy before he fell? b. What was Superman’s Kinetic Energy just when he hit the ground? c. What was the speed of Superman just when he hit the ground? 11. Spiderman, with a mass of 190 kg, watched, as Superman did all of the stuff above. He watched from the side of a building. He was 400 m above the ground. What was his potential Energy? 12. Spiderman thought it was funny when Superman shot the gun out of the bad guys hand. He started to laugh very hard. He laughed so hard that he fell off the side of the building. What was his Kinetic Energy and speed just before he hit the ground? 13. A pendulum, with a mass of 2 g, swings back and forth for a clock on a wall. The maximum height the pendulum reaches is 20 cm. a. What is the potential energy of the pendulum at the max height? b. What is the max kinetic energy? c. What is the speed of the pendulum? Key

1. 6543.8J 2. 1150W 3. a. 602.7J b. 5891.7J c. 3.27W 4. 1.3x105N 5. a. 4 b. 1.94 c. 48.5% 6. a. 1.74 b. 87% 7. 0.81m 8. KE=1.96x103J PE=0.103J 9. a. 1.96x105J b. 9.8x104J 10. a. -1.103x106J b. 1.103x106J c. 98.99m/s 11. 7.45x105J 12. -7.45x105J 88.56m/s 13. a. 3.92x10-3 J b. -3.92x10-3 J c. 1.97m/s