# Warm-Up Exercise for lecture 16

## Due 8:00 am, Thurs, 22 Oct 2009

Physics 105, Fall 2009

Enter your 3-digit class identification number:

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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.)
☐ yes ☐ no

A student sits on a stool which is free to rotate about its axis. She is holding weights in her hands. Her arms are extended, holding the weights as far from her body as possible. Someone gives her a push which starts her rotating. She then pulls the weights in close to her body. What happens to her moment of inertia as she pulls in the weights?
☐ increases
☐ decreases
☐ remains the same

What happens to her rotational speed as she pulls in the weights?
☐ increases
☐ decreases
☐ remains the same

What happens to her rotational kinetic energy as she pulls in the weights?
☐ increases
☐ decreases
☐ remains the same

Ralph ran across this website describing precession: http://science.howstuffworks.com/gyroscope1.htm. He watched the video with the bicycle wheel, but became very confused. He learned from the reading assignment that angular momentum is conserved, but it seemed to him that in this case the angular momentum of the wheel was constantly changing in direction as the wheel spun around. What could you say to him to relieve his confusion?

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?