SAT Subject Physics Practice Question 49: Answer and Explanation

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Question: 49

6. Three thin, uniform rods each of length L are arranged in the shape of an inverted U.

The two rods on the arms of the U each have mass m; the third rod has mass 2m. How far below the midpoint of the horizontal rod is the center of mass of this assembly?


Correct Answer: B


First replace each rod by concentrating its mass at its center of mass position.

The center of mass of the two m’s is at their midpoint, at a distance of L below the center of mass of the rod of mass 2m.

Now, applying the equation for locating the center of mass (letting y = 0 denote the position of the center of mass of the top horizontal rod), we find

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