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When a block is placed on a wedge as shown in the figure, the block starts sliding down and the wedge also starts sliding on the ground. All surfaces are rough. The centre of mass of the $(\text{wedge} + \text{block})$ system will move:

1. leftward and downward.
2. rightward and downward.
3. leftward and upward.
4. only downward.

Correct answer: rightward and downward.

Solution

The block and wedge exert internal forces on each other, so they cannot change the motion of the centre of mass of the combined system by themselves. Since the ground is rough, there is an external frictional force and gravity acts downward, so the CM moves downward and, in the given setup, rightward as the wedge-block system shifts.

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