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Q & A: Directions of a spinning toy top?

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Most recent answer: 10/08/2011
Q:
When you spin a toy top, it finally slows, slips and reverses it's direction. Why?
- Evelyn (age 61)
Bloomington, IN
A:
Wow! This question made me situp straight in my chair and think.  I vaguely recall a long time ago observing something like this but didn't pursue the issue.  I think the answer lies in the fact that there are two separate regimes that you have to consider.   In the first case the top acts like a gyroscope and it precesses due to the torque provided by the force of gravity on it.  You can calculate which direction it precesses by using the rate of change of angular momentum and its relation to an applied torque.  When the toy top topples, (I like the alliteration), the rules change more into the f = ma regime (don't worry, angular momentum is still conserved).  In this case the sliding friction forces between the still spinning top and the table are in the opposite direction of the original precession and the top starts skittering backwards.

LeeH

(published on 10/08/2011)

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