# Classical Mechanics-Spinning Top

1. Dec 4, 2013

### sclatters

1. The problem statement, all variables and given/known data

2. Relevant equations
torque=rxF
angular precession velocity=Δtheta/Δt
assume that Δtheta=ΔL/Lsin(theta)

3. The attempt at a solution
I can conclude that the subsequent motion of the top will be an anti-clockwise circle about the origin but would there be a bobbing motion? My answer for part a) just shows the spinning top making this circle. Do you think this is what the question is asking for?

For part b) I have worked out that the angular proccession velocity=mgd/Iω so would the frequency=mgd/2∏Iω? Does this answer sit alongside the statement of "using the variables given" even though there is no direct mention of I?

Last edited by a moderator: Dec 4, 2013
2. Dec 4, 2013

### Staff: Mentor

Yes.

The angular velocity of precession is the angular frequency.

No. Express I in terms of the variables given.

3. Dec 4, 2013

### sclatters

Great, thanks!

I am a little unsure on how to express I in terms of the variables given though?

4. Dec 4, 2013

### Staff: Mentor

What's the moment of inertia of a disk? You are given the mass and radius.

5. Dec 4, 2013

### sclatters

Of course, Ma2/4. Thank you very much for your help again!

Last edited: Dec 4, 2013
6. Dec 4, 2013

Almost.

7. Dec 5, 2013

### sclatters

I'm not sure why it isn't (ma^2)/4? Do I need to use the parallel axis theorem to find the moment of inertia in another position? Maybe at the point the spinning top intercepts the origin O?

8. Dec 5, 2013

### Staff: Mentor

Where does the 4 come from?

9. Dec 5, 2013

### sclatters

Sorry, I was thinking about the x and y components. They both equal (ma^2)/4 and the z component equals the x and y components added together. This gives the moment of inertia to be (ma^2)/2 straight through the disk as if it were spinning like a CD. Is this correct?

10. Dec 5, 2013

### Staff: Mentor

Yes, now you've got it.