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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?
- Saurav is wrong and Rahul is correct
- Saurav is correct and Rahul is wrong
- Both are correct
- 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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