Re: [Phys-l] conservation versus constancy

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

Well said John.  Thanks for the clarification.
Jeff

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of John Mallinckrodt
Sent: Tue 10/17/2006 6:47 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] conservation versus constancy



On Oct 17, 2006, at 2:54 PM, Bob LaMontagne wrote:

> I don't follow the claim about the work being zero in one frame 
> implies it
> is zero in all others.

Indeed, it doesn't.   It's easy to show that if the "center of mass 
work" (or "pseudowork" or whatever you want to call it, i.e., the 
integral of F_net dot dr_cm) done on an object in one inertial frame 
is W, then the work done in another inertial frame moving at velocity 
u relative to the first is

    W' = W - u dot delta_p

where delta_p is the change in momentum of the system.  The change in 
momentum is determined by the impulse which is frame invariant in 
Newtonian mechanics.  The change in kinetic energy, however, is not 
frame invariant in general nor, therefore, is the center of mass work.

In the particular case cited, the impulse is zero and in that very 
special case the work is frame invariant.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>



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