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Did you complete the reading assignment? (Typically this means at least 20-30
minutes looking over the assigned sections of the book.)
The total momentum of an isolated system of objects is conserved
only if conservative forces act between the
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?
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?
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