Laws of Motion
A plank with a rough lower surface is lying on a rough, horizontal surface. The upper surface of the plank is smooth and has a smooth hemisphere lying on it, hanging on a string from the ceiling, as shown in the diagram.
If the string is cut, the trajectory of the center of mass of the sphere will form
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1 solution
This is a pretty clever problem. When the string is cut, the only forces are vertical (gravity and normal reaction). Therefore, the center of mass can only move up or down, in a straight line.