spinning barbells

[Killjoy]

In the absence of a diagram my question may be a bit hard to follow, but I'll do my best...

 

Imagine I have a barbell floating in empty space - ie two equal weights connected by a rigid bar (a single rigid pole would do as well, but the barbell might be clearer).

I start it spinning on an axis that passes through the center of mass of the system (ie, middle of the bar) but is neither parallel nor perpendicular to to bar.

 

My intuitions as to what will happen lead in contradictory directions:-

1) Centrifugal force will cause the spinning barbell to straighten out so that the axis of spin becomes perpendicular to the bar;

2) Conservation of kinetic energy and angular momentum will mean that the barbell will continue to spin on the same axis as it started.

 

Which is right and why is the other one wrong?

[J.C.MacSwell]

In the absence of a diagram my question may be a bit hard to follow, but I'll do my best...

 

Imagine I have a barbell floating in empty space - ie two equal weights connected by a rigid bar (a single rigid pole would do as well, but the barbell might be clearer).

I start it spinning on an axis that passes through the center of mass of the system (ie, middle of the bar) but is neither parallel nor perpendicular to to bar.

 

My intuitions as to what will happen lead in contradictory directions:-

1) Centrifugal force will cause the spinning barbell to straighten out so that the axis of spin becomes perpendicular to the bar;

2) Conservation of kinetic energy and angular momentum will mean that the barbell will continue to spin on the same axis as it started.

 

Which is right and why is the other one wrong?

 

3) wobble

[Killjoy]

Originally Posted by Killjoy

In the absence of a diagram my question may be a bit hard to follow' date=' but I'll do my best...

 

Imagine I have a barbell floating in empty space - ie two equal weights connected by a rigid bar (a single rigid pole would do as well, but the barbell might be clearer).

I start it spinning on an axis that passes through the center of mass of the system (ie, middle of the bar) but is neither parallel nor perpendicular to to bar.

 

My intuitions as to what will happen lead in contradictory directions:-

1) Centrifugal force will cause the spinning barbell to straighten out so that the axis of spin becomes perpendicular to the bar;

2) Conservation of kinetic energy and angular momentum will mean that the barbell will continue to spin on the same axis as it started.

 

Which is right and why is the other one wrong?

[/quote']

3) wobble

 

Is this the same as (2)?

[J.C.MacSwell]

Is this the same as (2)?

 

It will continue (wobble about) on the same axis of rotation as it had instantaneously on the moment of release.

 

That axis is different from the one it was constrained to rotate about originally, and will depend on the original axis, and the moment of inertia about the bar axis relative to the moment of inertia about an axis perpendicular to the bar.

 

Of course, kinetic energy and angular momentum will be conserved.

Edited April 23, 2009 by J.C.MacSwell