# How does ball A come to rest and Ball B remain stationary?

## Homework Statement

In a collision between three identical steel balls A B C

A comes to rest and B remains stationary, while C rolls off.

In terms of the Forces acting, explain how Ball A came to rest and why Ball B remained stationary.

## The Attempt at a Solution

Well what I know here is that Ball A experiences the downward pull of gravity and also friction upon collision? Maybe that is why it stops.

I know that momentum is conseved and transfers through ball B but I can't think of the ''Forces acting here''

I do have the general idea of whats going on and that this collision is a good representation of an elastic collision where momentum and kinetic energy are conserved.

How can I represent this answer in a concise way without getting too complicated? Just the fundamental reasons

RUber
Homework Helper
Give ball A some velocity and mass.
Ball B and ball C have the same mass but zero velocity.
Assume an elastic collision.
Conserving kinetic energy gives something like:
##\left(v^2_A m_A + v^2_Bm_B+v^2_Cm_C\right)_{pre}=\left(v^2_A m_A + v^2_Bm_B+v^2_Cm_C\right)_{post} ##
Since all the balls are the same weight, you can factor them out of the equation.
##\left(v^2_A + v^2_B+v^2_C\right)_{pre}=\left(v^2_A + v^2_B+v^2_C\right)_{post} ##
Also, you can use conservation of momentum to get:
##\left(v_A + v_B+v_C\right)_{pre}=\left(v_A + v_B+v_C\right)_{post} ##
It's not very clean to solve all three together, so imagine a tiny gap between balls B and C...work out the relation from A to B (ignoring ball C), then the new B to C.
https://en.wikipedia.org/wiki/Elastic_collision

• Richie Smash
haruspex