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Hot Seat Review Game 1. The unit for work is Joules. What is an equivalent unit for work using meters, kilograms, and seconds? a) kg m/s2 b) kg m2/s c) kg m2/s2 d) kg2m/s2 c) kg m2/s2 2. If Zippy pushes a stationary brick wall ten times harder than he did yesterday, what happens to the work done? (Increase, Decrease or Stay the same.) Stay the same 3. An 8.0 kg mass is moving with a speed of 2.0 m/s. A 2.0 kg mass is moving with speed 4.0 m/s. Both objects encounter the same constant braking force and are brought to rest. How do their stopping distances compare? They are the same. ΔKE = W ½ 8(2)2 = Fd 16/F = d ΔKE = W ½ 2(4)2 = Fd 16/F = d 4. An object hits a wall and bounces back with one-third its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy? 1/9 5. At what rate is a 50 kg boy using energy when he runs up a flight of stairs 8 m high in 6 seconds? 653 W P = W/t P = ∆E/t P = mgh/t P = 50(9.8)(8)/6 6. A 10 kg mass is moving with a speed of 5.0 m/s. How much work is required to stop the mass? 125 J W = ΔKE = ½ 2 mv =½ 2 (10)(5 ) 7. You slam on the brakes of your car and skid a certain distance. If you had been traveling three times as fast, what distance would the car have skidded under the same conditions? Nine times farther. 8. An object that has kinetic energy must have: (mark all that apply) a) b) c) d) Momentum Acceleration A net force applied to it Potential Energy a) Momentum 9. An apple falls from a tree. Compare its kinetic energy to its potential energy as it falls. a) KE increases and PE decreases b) KE decreases and PE decreases c) KE increases and PE increases d) KE decreases and PE increases a) KE increases and PE decreases 10. A car moves twice as fast as a truck and has half the mass of the truck. What is the ratio of the car’s kinetic energy to the truck’s kinetic energy? 2 to 1 11. A lightweight object and a very heavy object are sliding with equal speeds along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height? They rise to the same height. 12. A 5.0 kg ball falls to the floor. When it is 0.80 m above the floor, its potential energy exactly equals its kinetic energy. How fast is it moving? 3.96 m/s 13. A ball falls from the top of a building through the air (air friction is present) to the ground below. How does the KE just before striking the ground compare to the PE at the top of the building? PEmax > KEmax 14. How many joules of energy are used by a 2.0 hp motor that runs for 1.0 hr? (1 hp = 746 W) 5,371,200 J 15. If a skater’s speed increases so he has twice the momentum, then his kinetic energy increases by what factor? Four 16. Consider molecules of hydrogen gas and molecules of heavier oxygen gas that have the same kinetic energy. Which molecules have a greater speed? hydrogen gas