much downward force are you exerting on the basket? a.15 Nb.more than 15 Nc.less than 15 Nd.more information is needed Answer (a) Equilibrium: force up = force down. Question 20 – When a moving hammer hits a nail, it exerts the enormous force needed to push the nail into wood. This force is far greater than the hammer’s weight. How can this be explained? Answers (b) and (c) are both correct (I realized this after the test, and and I graded accordingly: you only lost credit if you answered (a) or (d)). Eq. (2.3.2): impulse = force x time = change in momentum, so (force on nail) = (change in momentum of hammer) / (contact time). This gets big if either the change in momentum is big or the contact time is small, or both. Question 21 – You are playing a game of marbles on a sandy playground. The goal is to knock glass marbles out of a circle by hitting them with other marbles. You initially drop a marble onto the ground inside the circle and it hardly bounces at all. What prevents it from bouncing well here? Answer (c) See subsection “How the surface affects the bounce” starting on page 104 in book, and “Check your understanding #2: The game begins” Force on marble and on sand are equally big (Newton’s third). But the sand indents more, so receives most of collision energy. Sand has a negligible coefficient of restitution, so the bulk of the collision energy is lost into thermal energy.
PHYS115 – Spring 2013 9 Test 2 - 19 April 2013 Question 22 – The engine of a train you’re chasing has broken down, but it’s still rolling forward at 20 m/s. The train’s mass is 30,000 kg. To stop it, you grab onto the Iast car and begin to drag your boot heels on the ground. The backward force on the train is 600 N. How Iong will it take you to stop the train? Answer (d) 30,000 x 20 / 600 = 1,000 seconds. See “Check Your Figures #2: Stop That Train” on p. 79 in book.
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