### Solution to problems – class 9 – Set 1 Q 57

##### July 1, 2019

## Problem Statement

**57) A ball X of mass 1 kg travelling at 2 m/s has a head-on collision with an identical ball Y at rest. **

*X*stops and*Y*moves off.**Calculate the velocity of**

*Y*after the collision.## Solution

Initial momentum of X = 1×2 kg m/s = 2 kg m/s …………….(i)

After collision X stops and Y moves off, with say V m/s

Y is identical to X. So its mass is also 1 kg

So final momentum = 1 x V =V……………….(ii)

As per law of conservation of momentum,**V = 2**

Here we get the value of velocity of Y as 2 m/s