SAT Subject Physics Practice Question 170: Answer and Explanation

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Question: 170

16. It refers to the collision of two blocks on a frictionless table. Before the collision, the block of mass m is at rest.

If the collision were elastic, what is the total kinetic energy of the blocks just AFTER the collision?

A. 16 J
B. 32 J
C. 64 J
D. 128 J
E. 256 J

Correct Answer: D


Using the equation K = mv2/2, we can figure out that before the collision, the kinetic energy of the left-hand block was (4 kg)(8 m/s)2/2 = 128 J and that of the right-hand block was zero (since it was at rest), giving a total kinetic energy before the collision of 128 J. Since kinetic energy is conserved in an elastic collision, the total kinetic energy after the collision must also be 128 J.

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