A block is struck by a pendulum bob at the lowest point in its swing. How much faster will the block's initial speed be after being struck if the bob is swung from four times the height if the bob and the block have the same mass and the collision is perfectly elastic?
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2 times as fast
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16 times as fast
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4 times as fast
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the same speed
A
Correct answer
Explanation
The pendulum bob's speed at the bottom is given by v = sqrt(2gh). When the height is quadrupled, the speed only doubles (sqrt(4) = 2). In a perfectly elastic collision between objects of equal mass, the bob transfers all its velocity to the block, so the block's initial speed becomes twice as fast.