Question

A ball with a mass of `9 kg` moving at `7 m/s` hits a still ball with a mass of `15 kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

`4.2` `m/s` towards the direction the first ball was going previously.

Explanation:

I'm Considering the Collision is Elastic.

According to the Law of Conservation of Momentum,

`m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2` [The total momentum of

a system should be constant after a collision]

So, Substituting the values,

`9 xx 7 + 15 xx 0 = 9 xx 0 + 15 * x` [Let the required velocity be `x`.]

`rArr 15x = 63`

`rArr x = 63/15 = 4.2`

The velocity is positive, so the second ball will head towards the direction the first ball was initially going.

Hence explained.