# Does the classical theory of angular momentum explain this video of a unicycle robot?

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• Ricardo507
Ricardo507
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Does the classical theory of angular momentum explain the following video?
Hi. I'm an enthusiast of physics applied to robotics (you know, modeling and stuff), I've been studying a bit of unicycle robot and how the gyroscope theory helps us in these cases. However, I'm totally drawing a blank about how to explain in the video below:

Gyroscope theory tells me that that angular momentum vector should rotate and "Try" to align with the torque produced by the weight, however, the mechanism in the video can't do that (and isn't doing it at all). The torque produced by the weight is pointing in/out of the screen and the angular momentum of the wheel is parallel to it (I think). I don't understand why it doesn't fall.

Thank you everybody

sophiecentaur
If I understand what's going on correctly, then what's happening is that if the whole thing were balancing on a point, it would either fall over or rotate in a circle but as it actually is, the device has limited freedom of motion due to the fact that the base is spread over a line and the rotation isn't enough to shift that support base so it just stays upright.

Ricardo507 said:
Does the classical theory of angular momentum explain the following video?
Do you mean the first example with balancing on one edge?
Ricardo507 said:
Gyroscope theory ...
Not sure what you mean by "gyroscope theory", but precession is not possible here, because the support is a horizontal line (hinge joint) not a point (ball joint). You seem to be overthinking what is effectively simple 2D mechanics, in the plane perpendicular to the support line and flywheel axis of rotation.

Last edited:
Ricardo507 said:
Gyroscope theory tells me that that angular momentum vector should rotate and "Try" to align with the torque produced by the weight, however, the mechanism in the video can't do that (and isn't doing it at all).
I believe that we should discuss that a little more.
A precession moment only happens when the plane of rotation of the wheel if forced to change orientation.

Each accelerating wheel induces a moment at its axis, which counteracts the falling moment that gravity induces about the support line or spherical pivot.

It seems to me that the off-verticality corrections are so quick that any precession moment induced by the counter-part wheel is barely noticeable or so weak that the friction at the spherical pivot is able to prevent it from developing.

In the following video, please note how big perturbations, induce certain rotation about the vertical axis.

There is nothing gyroscopic about this video. It is purely inertial acceleration or deceleration about an axis in order to manage that axis. The other wheel is not even needed. The device will react opposite to the wheel acceleration or deceleration (reverse acceleration).

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