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Unformatted text preview: ant speed of 2 m/s. How hard must the si des of the blender push inward on 0.008 kg of the spinning water? a.
d. 0.16 N 1.6 N 0.32 N 0.0016 N ANSWER: speed
22 centripetal force mass mass 0.008 0.32 N mass radius
0.10 acceleration 2 2 Question 14 – Your car has a mass of 1000 kg. It is movin at a speed of 4 m/s when it strikes a ng
parked car. Your car stops in 0.2 s. What force did the park ed car exert on it to stop it that quickly? k a.
d. 20 kN 5 0 N
200 N it depends on the mass of the parked car PHYS115 - Fall 2012 7/8 Test 2 - November 28, 2012 Before collision, car’s momentum is mass speed = 1,000 4 = 4,000 kg∙m/s ; equation (2.3.1) on page 88. After collision, speed is zero so momentum also zero. Car changes momentum from 4,000 to 0, so change in m omentum is 4,000 kg∙m/s. It changes momentum because an impulse acts on it (pa ge 77); this is force x time ; equation a
(2.3.2) on page 89. Change in momentum = force x time, and so 4,000 = force? 0.2, and so force = 4,000 / 0.2 = 20,000 newtons = 20 kN. Question 15 – You take the elevator from the 3rd floor to the 9th floor. Between the 8th and the 9th floor the elevator slows down so it can stop at the 9th floor. An accelerometer is mounted to the ceiling of the car. Which picture shows the accelerometer between the 8th and the 9th floor? h v v v Answer (d): Cannot tell because the speed is unknown (a) (b) (c) (d) Answer: Between 8th floor and 9th floor, elevator changes velocity from (up, larger) to (up, smaller), so y
acceleration is down. The block in the accelerometer mu st also accelerate down, so it needs downward forces to act on it. This happens in picture (A ): the top spring wants to expand (down force), and the bottom spring wants to contract (d own force). PHYS115 - Fall 2012 8/8 Test 2 - November 28, 2012 Question 16 – Suppose you take off in a car with your physics book on top. If you are accelerating forward and the book rides with you, in what direction does friction act on the book? a.
d. backwards forwards up (in the dir...
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