A ball of mass m approaches wall of mass M ( m) with speed 4 m/s along the normal to the wall. The speed of wall is 1 m/s towards the ball. the speed of the ball after and elastic collision will be:
1)5m/s 2)9m/s 3)3m/s 4)6m/s.

Question

A ball of mass m approaches wall of mass M (>> m) with speed 4 m/s along the normal to the wall. The speed of wall is 1 m/s towards the ball. the speed of the ball after and elastic collision will be:
1)5m/s 2)9m/s 3)3m/s 4)6m/s.

abhisar · Answer

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abhisar · Answer

Let us assume that the wall is moving towards right and the ball is moving towards left. Relative velocity of wall wrt to the ball will be 1-(-4)=5m/s. After collision let v be the velocity of the ball towards right, then the relative velocity of wall wrt to the ball will be (1-v) m/s [Velocity of wall will be unchanged after the collision because M>>m]

abhisar · Answer

Now using conservation of linear momentum,

5 + (1-v)=0
=>v=6 m/s