Re: [Phys-l] heat +- impulse

[John Denker <jsd@av8n.com>]

On 11/04/2007 12:13 AM, LaMontagne, Bob wrote:

> If I stand on roller skates and push against a wall, an impulse acts
> for a while, resulting in my having momentum - but then the impulse
> is gone. The only question is whether or not a state variable has
> been created. The total momentum of the earth and myself was zero
> before and after the push. So did the impulse cause a change in a
> state variable? Interesting!


If we take the skater to be the "system", then yes, the
momentum of the skater is a state variable.  It is a function 
of state for the skater.  We just transferred some momentum
across the boundary of the system.  In the formalism of 
thermodynamics, this is analogous to transferring energy 
across the boundary of the system.  Energy transfers may be
more-widely discussed in the textbooks, but it is no big
deal to handle momentum transfers on the same footing.

As an important example, the Euler equation for fluid flow 
is best thought of as a continuity equation for the flow of 
momentum.
  http://www.av8n.com/physics/euler-flow.htm