Re: [Phys-l] basic laws of motion +- vectors +- angular momentum

[John Denker <jsd@av8n.com>]

In the context of:

> > 2) Conservation of angular momentum implies conservation of linear
> > momentum ... but not vice versa.  Hint:  consider angular momentum
> > about an axis infinitely far away.

On 12/09/2008 04:15 PM, Robert Carlson wrote:
> I'm not sure number 2 is correct.  Consider a uniform rod with two
> forces, equal in magnitude, applied at the ends and perpendicular to
> the central axis of the rod.
>
> If the forces act in the same direction, then angular momentum will
> be conserved about the center of mass, but linear momentum will not
> be conserved.

Bad choice of axis.  Just because it can't be proved
using that axis doesn't mean it can't be proved.

Take another look at the hint.

> If the forces are applied in opposite directions (a force couple)
> then linear momentum will be conserved about the center of mass, but
> angular momentum will not be conserved.

Agreed.  That's the central theme of this thread.