Interesting fact about a wheel.

[Badjelly]

Made a drawing.

Why is the curved blue line with arrows on the end in the right-hand panel curved? This suggests the wheel is tilting as it moves from side to side, which is quite an important detail.

[GSVR]

Why is the curved blue line with arrows on the end in the right-hand panel curved? This suggests the wheel is tilting as it moves from side to side, which is quite an important detail.

Its showing that it can rotate about that axis. Could do the full revolution if it wanted.

[GSVR]

There's your answer.

And if I started it spinning in the cradle as well as the wheel spinning?

[Mikkel]

And if I started it spinning in the cradle as well as the wheel spinning?

That's quite different. Now you're talking about applying a torque that'll try to change the direction of the axis...

Around which axis do you want to spin the cradle? The vertical?

If so you'll find that the cradle won't spin - but the wheel will tilt.

See this video:
[YOUTUBE]http://www.youtube.com/watch?v=dCcfKBfmyP4[/YOUTUBE]

[Badjelly]

The rotation is important. If you get a spinning wheel (doesn't matter how fast) and push it around without changing the direction of the axis of rotation then it will behave just like a body of the same mass that's not spinning. As soon as you try to change the direction of the axis of rotation (or the rotation rate) then you bring angular momentum effects into play. These can be very strong and they work in surprising directions, as anyone who has played with a gyroscope will know.

[GSVR]

The rotation is important. If you get a spinning wheel (doesn't matter how fast) and push it around without changing the direction of the axis of rotation then it will behave just like a body of the same mass that's not spinning. As soon as you try to change the direction of the axis of rotation (or the rotation rate) then you bring angular momentum effects into play. These can be very strong and they work in surprising directions, as anyone who was played with a gyroscope will know.

Yes the axis it can rotate in train example is the same as steering a bike except horizontal instead of vertical. And no rake or trail.

[Mikkel]

Honesty I get tired of people that post youtube video as I can't see it on dailup.

Patience is a virtue!

Honestly, I think the mods get tired of people quoting embedded media as they have to correct such mistakes.

Yes the axis it can rotate in train example is the same as steering a bike except horizontal instead of vertical. And no rake or trail.

Honestly, I get tired of trying to explain how classical mechanics work as it seems people either already know or don't want to listen anyway.

What Badjelly said is correct.

If you know vector calculus wikipedia will be able to help you:

Classical Mechanics

Angular momentum

Frame of reference

Cheers
Mikkel

Case in point:

I also don't need to see static gyroscopes if the thing was flying trough space at several thousand kph that would be interesting.

See FoR...

[GSVR]

I can't agree with you here, Mikkel. I think its undeniable that, in a stationary frame of reference, the top half of a rotating wheel has more forwards momentum than the bottom half. To prove this, just integrate the product of mass and velocity over each half.

As you say, it's a rigid system, so pretty quickly the top half will become the bottom half (for what that's worth). And I'm curious, as you are, about what implications GSVR thinks this difference in momentum has.

But, does the top half have more forward momentum than the bottom at any instant? Yes. Can we move on?

I say that a static spinning wheel behaves differently to one that is moving. ie the center of rotation is moving. In my case its moving at 100 kph same as the surface speed of the tyre.

[Badjelly]

OK, train moving at 100 km/h on tracks, cradle on train with wheels that let it move sideways (ie perpendicular to train tracks), wheel on crade mounted so it can tilt (still not 100% what directions it can tilt in). What now?

[GSVR]

OK, train moving at 100 km/h on tracks, cradle on train with wheels that let it move sideways (ie perpendicular to train tracks), wheel on crade mounted so it can tilt (still not 100% what directions it can tilt in). What now?

It can revolve so the bottom becomes the top and the top the bottom. Which effectively reverses the direction of rotation. But the axle is alway perpendicular to the direction of travel of the train carraige.

[Badjelly]

I say that a static spinning wheel behaves differently to one that is moving. ie the center of rotation is moving. In my case its moving at 100 kph same as the surface speed of the tyre.

I think that's probably wrong, but before I can say for sure, I need to know what you mean by "behaves differently".

Einstein used to explain some of his arguments as "thought experiments". Here's a thought experiment: you're in a box that is either stationary or moving at constant velocity, you don't know which. (Think of it as an aeroplane, if you wish.) Can you, by doing experiments with (say) a spinning bicycle wheel, tell whether you're moving and how fast? The answer is no.

[Badjelly]

It can revolve so the bottom becomes the top and the top the bottom. Which effectively reverses the direction of rotation. But the axle is alway perpendicular to the direction of travel of the train carraige.

OK, I think I've got that.

[Mikkel]

I can't agree with you here, Mikkel. I think its undeniable that, in a stationary frame of reference, the top half of a rotating wheel has more forwards momentum than the bottom half. To prove this, just integrate the product of mass and velocity over each half.

As you say, it's a rigid system, so pretty quickly the top half will become the bottom half (for what that's worth). And I'm curious, as you are, about what implications GSVR thinks this difference in momentum has.

But, does the top half have more forward momentum than the bottom at any instant? Yes. Can we move on?

Sorry, I forgot that one...

Yes true - the upper part has a forward momentum of X and the lower half has backward momentum of X. They cancel out and there is no momentum due to the rotation of the mass.
If the centre of mass moves - then there's a momentum equal to the product of the speed and the mass.

What you have for a stationary rotating wheel is an angular momentum equal to the product of the angular frequency and the rotational inertia.

My point is - if you want to consider a wheel from a mechanical perspective it makes no sense to treat the upper and the lower part of the wheel as two seperate entities. It's ONE wheel and should be treated as such.

[GSVR]

OK, I think I've got that.

I'll turn this around a bit and say which direction would the wheel be the most stable. Turning like it would going down the road or in reverse.

Or maybe it doesnt matter. ( By saying this I'm going against what I believe to be true)

[Mikkel]

I'll turn this around a bit and say which direction would the wheel be the most stable. Turning like it would going down the road or in reverse.

Or maybe it doesnt matter.

The fact that the train is moving doesn't matter as long as it's a steady motion(constant velocity).

The direction the wheel turns does not impact on it's stability - only the direction of the angular momentum.

Have you read anything posted in this thread or followed any of the links that has been provided?