Question

A ball with a mass of `5kg` moving at `4 m/s` hits a still ball with a mass of `7 kg`. If the first ball stops moving, how fast is the second ball moving?

Answer 1

I found: `2.8m/s`

Explanation:

We can try using conservation of momentum `vecp=mvecv` along, say, the `x` axis:

`p_("before")=p_("after")`

With your data:

`(5*4)+(7*0)=(5*0)+(7*v)`

`7v=20`

`v=20/7=2.8m/s`