# Momentum with billards balls.

• mitchmcsscm94

## Homework Statement

Two billiards balls of equal mass move at right angles and meet at the origin of an xy coordinate system. One is moving upward along the y-axis at 2.0 m/s, and the other is moving to the right along the x-axis with speed 3.7 m/s. After the collision( assumed elastic), the second ball is moving along the positive y axis. What is the final direction of the first ball, and what are the two speeds?

## Homework Equations

it is elastic so
P1+P2=P1final+P2final
mass is constant so it cancels
V1+V2=V1final+V2final

## The Attempt at a Solution

i got 5.7=V1final+V2final

A couple of things that might help:

Since the collision is elastic, kinetic energy is conserved.
The problem is in two dimensions! Remember momentum is a vector quantity. (Can you just add the initial velocities to give 5.7ms^-1 as you have done?)

i did the KE+KE=KEf+KEf
I ended up with basically the same thing only the magnitude is squared. would the first ball come to rest and give ball 2 all its energy? that would mean that ball 2 would have a vfinal of 5.7 m/s. would that work? I am studying for a big test monday in AP Physics and this question has me beat.

You missed my second point. Since the collision is in two dimensions, you have two equations for conservation of linear momentum- one in the x-direction, and one in the y-direction.
Of course KE is only concerned with the magnitude of the velocity, so there is no need to resolve your KE equation into components.