Motorized top with internal unbalanced flywheel

[Russ Edmonds]

TL;DR

The physics of how a motorized top with internal unbalanced flywheel works seems to be in debate.

Finished version 3 of a motorized top with unbalanced flywheel. Used a larger more efficient motor and two AAA Ni-Cd 1.2 volt 150 mAh cells in series for power. The question is how does this top work?

Before commenting, please carefully read the following.

The top can be thought of as two rigid bodies. One is the circular symmetric outer shell which is connected to the outer case of the motor and the tip of top that makes contact with a horizontal surface. The second is a statically unbalanced flywheel attached to shaft of the motor. The motor shaft is co-linear with the symmetry axis of the outer shell.

The flywheel must be statically unbalanced, a balanced flywheel does not work.

The top is spun clockwise by hand, when viewed from above and the flywheel is spun counterclockwise by the motor. If the flywheel is spun clockwise by the motor, the top does not work.

When I turn on the top the flywheel spins up to some max angular speed based on the battery voltage. The motor torque applied to the flywheel is equal and opposite to that applied to the outer shell. The outer shell remains stationary because I'm holding it. The top will not stand up without spinning the outer shell. This shows that the angular momentum vector of just the counterclockwise spinning flywheel is not enough for the top to stand up. When I give the outer shell a good clockwise spin it stands up and precesses clockwise. The angular momentum of the outer shell is now much larger than that of the flywheel. The Limbo top and my top both precess in the same direction as the spin of the outer shell proving that the total angular momentum is in the direction of the outer shell.

The physics of how tops like this work seems to be in debate. The explanation given here for example

is incorrect for my top at least, because experiments show the gravitational torque discussed is insignificant.

I placed the top with flywheel spinning at full speed on a floating platform with the outer shell attached to the platform and the flywheel spin axis at a 45 deg angle with vertical. If the gravitational torque discussed in the video is correct the platform should rotate. It was found that the platform did not rotate.

 

[wrobel]

Beyond different speculations and pop-physics, classical mechanics is a complicated nonlinear science full of counterintuitive effects. To understand the behavior of the system, one must write down the equations of motion and analyze them correctly. This may occur to be a hard task.

 

[A.T.]

The physics of how a motorized top with internal unbalanced flywheel works seems to be in debate.

Seems similar to plate spinning, but with energy input at the other end of the support.
https://en.wikipedia.org/wiki/Plate_spinning

 

[Russ Edmonds]

You might be right. Here is a high speed video of the tip movement viewed from below. It shows the tip making small counterclockwise circles due to the unbalanced flywheel.