Question

# A prticle of mass m moves with velocity v0=20 m/s towards a large wall that is moving with velocity v=5 m/s towards the particle as shown. If the particle collides with the wall elastically, then find the speed of the particle just after collision. (Assume collision with the wall is elastic)

A
30 m/s
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B
20 m/s
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C
25 m/s
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D
22 m/s
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Solution

## The correct option is A 30 m/sAs we know e=v2f−v1fv1i−v2i (Take positive motion towards right) Here, v1i= Speed of particle before collision =v0 v2i= Speed of wall before collision =−V v2f= Speed of wall after collision =−V v1f= Speed of particle after collision =−v′ Assume after the collision, particle will move in leftward direction. Hence, 1=−v−(−v′)v0−(−v) vo+v=−v+v′ v′=2v+v0 v′=2×5+20 v′=30 m/s Therefore, speed of the particle after collision is v′=30 m/s towards left.

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