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Honors Physics Unit 5 Review Guide – Work and Energy Test Format 10 multiple choice problems (2 points each) 5 short problems (6 points each) 1 explanation problem (10 points) 1 bar chart problem (10 points) 2 medium problems (15 pts each) Topics Covered Energy o Various types of energy o Kinetic energy o Gravitational potential energy Choosing a baseline o Elastic potential energy Work o Relationship with force and distance o Perpendicular forces do no work o Sign of the work Work – Energy o Connection between work and kinetic energy o Doing work more KE o Negative work less KE Conservation of Mechanical Energy o No frictional forces KE + PE conserved o Identifying initial and final states Practice Problems Multiple Choice: 1. Which of the following situations is there positive work being done? a. lifting an object b. dropping an object c. pushing an object d. all of the above 2. If I were to double the velocity of an object, what happens to its kinetic energy? a. doubles b. quadruples c. halves d. quarters 3. I throw a ball into the air. When does its potential energy reach a maximum? a. when it leaves my hand b. at the top of the trajectory c. when it reaches the ground d. potential energy is same everywhere 4. Two people throw an object from the same height with the same speed. Object A is thrown up and object B is thrown down. Which one is moving faster right before it hits the bottom? a. Object A b. Object B c. same time d. depends on the masses of the object 5. A person swings on a pendulum. What is the correct explanation for the height getting slightly smaller and smaller during each swing? a. You keep turning potential energy into kinetic energy in each successive swing thereby losing height each time b. Gravity is doing negative work on the system, thereby causing a decrease in kinetic energy and therefore a decrease in height c. There is friction and/or air resistance present which causes a small loss of mechanical energy during each swing. d. IT’S A LIE. Mechanical energy is always conserved and the pendulum will swing at the same height FOREVER. Short Problems: 6. A 2 kg object is 10 meters in the air and moving to the left at 1 m/s. What is its potential and kinetic energies? 7. A platform diver has a mass of 50 kg. She steps off a diving board that is elevated to a height of 10 meters above the water. How much kinetic energy will the diver have right before she hits the water? 8. A toy gun uses a spring with a spring constant of 40 N/m that is compressed 0.15 meters. How fast can it launch a 0.5 kg projectile? 9. Calculate the work required lift a 2.5-kg object to a height of 6.0 meters. 10. A 10 kg block is sliding along a rough ground with a constant force of friction of 100 N. If it starts with a speed of 20 m/s, how far does the block travel before stopping? Explanation Problem 11. Describe the energy transformations that occur while an object is loaded onto a spring which is compressed and then fired up into the air before landing back on the ground. Bar Chart Problem 12. Draw a bar chart representing the energies at the various stages of the object described in the previous problem. The actual numerical values are not required, just the relative sizes of the bars. Draw the bar charts for the kinetic energy, elastic potential energy and gravitational potential energy at the very beginning, the top of the motion and right before it hits the ground Medium Problems: 13. A ball with a mass of 2 kg starts at rest 3 meters above the ground and then slides down a smooth curved track before going back up the other side a. (2 pts) What is the initial kinetic energy? b. (3 pts) What is the initial potential energy? c. (6 pts) What is the speed of the ball at the bottom of the track? d. (4 pts) How far up the other side of the track does the ball travel? 14. A 800 kg car travelling at a constant speed, slams on its breaks and comes to a stop over a distance of 25 m. The force of friction between the car and the road is a constant 2000 N. a. (4 pts) What is the work done by friction while the car is stopping? b. (6 pts) Using the work-kinetic energy theorem, determine the initial speed of the car. c. (5 pts) If the initial speed of the car were twice as fast as you got in part b), what would be the new stopping distance of the car? Practice Problems Key 1. 2. 3. 4. 5. D B B C C 6. PE = 196 J, KE = 1 J 7. PE = 4900 J so the final KE = 4900 J 8. v = 1.34 m/s 9. W = 147 J 10. d = 20 m 11. Starts with elastic potential energy because spring is compressed Turns into kinetic energy as it leaves the spring Kinetic energy turns into potential energy as the object gains height until reaching a maximum Potential energy turns back into kinetic energy as the objects falls to the ground 12. KE PEg PEe Initial 13. a. b. c. d. Top Ground 0 58.8 J v = 7.67 m/s goes back up to same height = 3 m 14. a. W = -fd = -50,000 J b. v = 11.2 m/s c. stopping distance would quadruple = 100 m