Re: [Phys-l] Rocket Hovering and Conservation of Momentum

[John Denker <jsd@av8n.com>]

Roger Haar wrote in part:

> 	I would suggest that when solving a problem, if one's first attempt at 
> understanding and solving it indicates that momentum is not conserved, 
> then one needs to consider adding something to your chosen system.

That's not wrong.  That leads directly to a _global_ conservation law.

However, there are additional options that may be more convenient and/or
more informative.

In particular, it is important that we have a _local_ law of conservation
of momentum.  It states that for _any_ region (not just ultra-large regions)
the momentum in that region does not change except by flowing across the
boundary.

The local law is more powerful;  the local law implies the global law and
*not* vice versa.