# Warm-Up Exercise for lecture 12

## Due 8:00 am, Thurs, Oct 9

Physics 105, Fall 2008

A truck always has more mass than a roller skate. Does a truck always have more momentum than a roller skate?
☐ yes
☑ no

The total momentum of an isolated system of objects is conserved
☐ only if conservative forces act between the objects
☑ regardless of the nature of the forces between the objects.

A ping-pong ball moving forward with a momentum p strikes and bounces off backwards from a heavier tennis ball that is initially at rest and free to move. The tennis ball is set in motion with a momentum:
☑ greater than p
☐ less than p
☐ equal to p

Ralph asked me a question about today's reading assignment. Suppose he is floating in outer space with no forces acting on him. He is at rest, so his momentum is zero. Now, he throws a ball. The ball goes one way, and he goes the other way. Before the collision, there was no momentum, and after the collision, there is plenty of momentum. He has momentum, and the ball has momentum! It certainly doesn't look like zero momentum, so it doesn't look like momentum was conserved. How would you answer Ralph?

Ralph needs to remember that momentum is a vector. Thus, although momentum certainly exists after he throws the ball, there is no *net* momentum because e.g. he has positive momentum but the ball had negative momentum.