# Conservation of kinetic energy and momentum in a collision

1. Jun 21, 2009

### sugaJ

A white billiard ball collides with a stationary red billiard ball of equal mass. The white ball stops and the red ball moves off with the same initial velocity as the white ball. Which of the following statements about a collision between two billiard balls is correct?
a) As both momentum and kinetic energy are conserved, the collision is elastic.
b) As neither momentum nor kinetic energy are conserved, the collision is inelastic.
c) As momentum is conserve, but kinetic energy is not, the collision is elastic.
d) As kinetic energy, but not momentum is conserved, the collision is inelastic.

I am pretty sure that momentum is conserved. I don't think kinetic energy would be conserved (as some would be lost to sound energy etc) but if the ball leaves at the same velocity, my logic is telling me that all kinetic energy must be conserved and therefore the answer is a, but I'm not sure. It would be greatly appreciated if someone could help me out.

Cheers

2. Jun 21, 2009

### Staff: Mentor

Good. Momentum will always be conserved in a collision.
You are told that the velocity is the same, thus the final KE must equal the initial--KE is conserved. So yes, the answer is a.

But you are also correct that in reality some energy will be "lost" as sound energy, but that is a small effect typically ignored in these kinds of problems. Good thinking, though.

3. Jun 21, 2009

### sugaJ

awesome thanks heeps mate.