# Warm-Up Exercise for lecture 16

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

Physics 105, Fall 2009

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. If she then pulls the weights in close to her body, she will rotate faster. What happens to her moment of inertia as she pulls
in the weights?

☐ increases

☐ 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?

Angular momentum is NOT conserved here. There IS an external torque: the support string. In this case, the change in angular momentum is illustrated by a change in DIRECTION, rather than a change in SPEED.

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