# Warm-Up Exercise for lecture 11

## Due 8:00 am, Tues, 6 Oct 2009

Physics 105, Fall 2009

Enter your 3-digit class identification number:

Enter the last 4 digits of your BYU ID:

Did you complete the reading assignment? (Typically this means at least 20-30 minutes looking over the assigned sections of the book.)
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 truck always has more mass than a roller skate. Does a truck always have more momentum than a roller skate?
yes
no

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?

The comments in the next next two boxes go into a big, mostly anonymous text file that I skim through before the morning lecture, using the comments to help me plan class discussion. ("Mostly anonymous", because I can track down who made what comment, but it takes some effort on my part to do so.) Therefore, if you really want to make sure I see your question/comment and answer it individually, you should send it to me via email and not through this form.

Which part of today's assignment was particularly hard or confusing? What would you like to spend extra time on in class?