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Unformatted text preview: mass is forced to stay with the elevator, the acceleration of the block in the accelerometer must also be up, and so must the force acting on it. This is the case in panel (a); the bottom spring wants to e xpand (restoring force is up), and the top one wants to contract (again, restoring force is up). (You could also argue that the apparent weight is down and is larger than t he gravitational weight m x g.) Question 12 – You’re driving in your car at 60 mph. You press the brake, and lower your speed to 30 mph. Your kinetic energy is now … a. … 50% of what it was before b. … 25% of what it was before c. … 12.5% of what it was before d. it depends on how long you used the brake Answer (b) 2
Kinetic energy = ½ x mass x speed = ½ x mass x speed x speed. Here the sp eed goes down by a factor ½, so the kinetic energy goes down by a factor ½ x ½ = ¼, so 25%. PHYS115 – Spring 2013 6 Test 2 - 19 April 2013 Question 13 – You are flying on an airplane and you notice th at a ball of aluminum foil from lunch starts to roll to the left. The motion of the foil ball implies that the p lane is … a. … moving at constant v elocity toward the right b. … turning toward the ri ght c. … moving at constant v elocity toward the left d. … turning...
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