FIGURE 9-38 shows a block of mass 2m at rest on a horizontal, frictionless table. Attached to this block by a string that passes over a pulley is a second block, with a mass m. The initial position of the center of mass of the blocks is indicated by the point i.

The blocks are now released and allowed to accelerate; a short time later their center of mass is at the point f.
(a) Did the center of mass follow the red path, the green path, or the blue path?
(b) Choose the best explanation from among the following:
I. The center of mass must always be closer to the 2m block than to the m block.
II. The center of mass starts at rest, and moves in a straight line in the direction of the net force.
III. The masses are accelerating, which implies parabolic motion.

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