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

[Rick Tarara <rtarara@saintmarys.edu>]

----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>


> Quist, Oren wrote:
>
> > Momentum is conserved only if there are no external forces.  There is an
> > external force on the rocket ship (gravity) -- don't expect momentum to
> > be conserved for the rocket ship.
>
> Pleeeease don't say that.  That is at best an abuse of the terminology,
> and is very likely to spread misconceptions.
>
> There is a crucial difference between /constancy/ and /conservation/
>
> a) Constancy means simply that the momentum in a given region doesn't 
> change.
>
> b) Conservation means that the momentum in a given region doesn't change
>    _except by flowing across the boundary_.
>
>
> Momentum is *always* conserved.  Always.  This is one of the grandest and 
> most
> fundamental principles of physics.  You should never pass up an 
> opportunity
> to teach this point.
>
> There is no law of constancy of momentum ... although you can find special
> situations where the momentum is constant, for example if there is no 
> momentum
> flowing across the boundary.

I have a nuclear powered aircraft-carrier sitting at rest in a calm sea.  I 
start up the reactor, the boilers, and start moving off.  I can follow the 
energy, and can describe the forces, but I'm having a harder time following 
the momentum here--maybe, just maybe, why most texts don't really talk about 
momentum conservation.  ;-)

Rick