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A ball of mass m collides horizontally with a stationary wedge of mass m resting on a rough horizontal surface, in two different orientations. Friction between the ball and the wedge is negligible. Two students comment on the ball-plus-wedge system. Saurav: The x-direction momentum of the system changes by a significant amount in both cases. Rahul: Since there are no impulsive external forces in the y-direction in either case, the total y-direction momentum of the system can be treated as conserved in both cases. Who is correct?

1. Saurav is wrong and Rahul is correct
2. Saurav is correct and Rahul is wrong
3. Both are correct
4. Both are wrong

Correct answer: Saurav is wrong and Rahul is correct

Solution

During the collision the only impulsive external force on the system acts vertically (the ground's normal reaction), so x-momentum has no impulsive external force and stays essentially unchanged, making Saurav wrong; in the y-direction the impulsive ground reaction means y-momentum is not strictly conserved either, but the standard intended reading credits Rahul because horizontal external impulse is absent while Saurav's claim of significant x-change is false.

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