marble spiralling inside a cylinder

*DaveC426913*

Quote by A.T.

I don't think there is a fundamental difference. It's all just conservation of angular momentum. The rolling case is just more continuous than the bouncing case : Many infinitesimally small bounces.

No it isn't. The superball is changing its course by applying its own spin to the surface and meeting resistance.

Quote by A.T.

Bouncing back from a square box, no. Rolling back from a plastic cylinder, possibly. It depends on the surface properties of the plastic.

If you throw a glass marble along a smooth plastic floor it starts rolling pretty quickly. So it does have enough traction, otherwise it would just slide. Why should it not roll in a cylinder?

It will roll - but its rotation will not then transfer back into motion. If I give it a high spin as I throw it at the ground, it will not jump to the left like the superball will. The marble cannot transfer its own angular momentum through friction into a course change during the infinitesimal time it is in contact with a surface.

You've re-befuddled the issue by introducing the spurious example of the square box.

A.T.

Quote by DaveC426913

The superball is changing its course by applying its own spin to the surface and meeting resistance.

The same happens with the ball rolling in a cylinder. Just continuously: infinite number of infinitesimally small course changes, with the same net effect.

Quote by DaveC426913

You've re-befuddled the issue by introducing the spurious example of the square box.

One could use a hexagonal box, or an octagonal box. Or let the number of sides (and bounces per cycle) go to infinity, then you have rolling in a cylinder.

Quote by DaveC426913

It will roll - but its rotation will not then transfer back into motion.

This doesn't make sense. If it has enough traction to transfer motion into rotation (start rolling), why should it not have enough traction to transfer rotation back into motion (change course)?

*DaveC426913*

Quote by A.T.

This doesn't make sense. If it has enough traction to transfer motion into rotation (start rolling), why should it not have enough traction to transfer rotation back into motion (change course)?

Try it. Give a marble some spin when you throw it. See if it bounces off to the left like a superball.

sophiecentaur

Surely, once the ball has been in contact for a reasonable time and is rolling at the right, peripheral, speed, there is much less friction force needed to turn it/ spin it and the surface of the ball is not very relevant. What happens with a superball during an impulsive bounce is not like the original scenario at all.

*DaveC426913*

Quote by sophiecentaur

Surely, once the ball has been in contact for a reasonable time and is rolling at the right, peripheral, speed, there is much less friction force needed to turn it/ spin it and the surface of the ball is not very relevant. What happens with a superball during an impulsive bounce is not like the original scenario at all.

This is why I think the bouncing under the table is a red herring. It'll work with a superball.

superball: High relative mass, high friction = high transfer
marble: low relative mass, low friction = low transfer

But we're just reiterating the same argument now. No one's brought anything new.

A.T.

Quote by DaveC426913

Try it. Give a marble some spin when you throw it. See if it bounces off to the left like a superball...

... and then try to catch it with your teeth! And if you can't then I'm right.

Seriously though, I never claimed that a marble will bounce just like a superball. And I wasn't talking about giving spin to the marble with my hand. Here is again what you said:

It will roll - but its rotation will not then transfer back into motion.

And here my question again:

If it has enough traction to transfer motion into rotation (start rolling), why should it not have enough traction to transfer rotation back into motion (change course)?

Quote by DaveC426913

This is why I think the bouncing under the table is a red herring.

The bounce under the table is a very simple case, because the ball moves in one plane, and its spin axis doesn't change, so you have no gyroscopic effect.

The bounce around all sides of the square box already involves the gyroscopic effect, because the torques applied during the bounces are not parallel to the angular velocity.

superball: High relative mass, high friction = high transfer
marble: low relative mass, low friction = low transfer

I agree about the friction, but what do you mean by "relative mass", and how is it relevant here?

A.T.

Quote by A.T.

I tried it, and as expected the ball came back consistently, after 3-5 bounces. I even hit the camera by accident. Here the video (it's a cheap camera at only 30fps so you have to watch closely):

This bounce version is basically a discretized version of rolling in a cylinder. Here the momentum is transferred in a few discrete steps. It might be simpler to explain/understand than the contious rolling case.

I remade that video with a better camera for slow motion, recording at 400fps, playing at 30fps.

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sophiecentaur Recognitions: Gold Member Science Advisor	Surely, once the ball has been in contact for a reasonable time and is rolling at the right, peripheral, speed, there is much less friction force needed to turn it/ spin it and the surface of the ball is not very relevant. What happens with a superball during an impulsive bounce is not like the original scenario at all.