# Already graded

# Warm-Up Exercise for lecture 16

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

Physics 105, Fall 2009

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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?

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