Re: ENERGY WITH Q

[Bob Sciamanda <trebor@VELOCITY.NET>]

The RHS of the CM work-energy system is the change in the CM kinetic
energy (1/2 the system mass times the square of the CM speed).

The KE of a mechanical system can be resolved into this (CM) kinetic
energy plus the kinetic energy of the system constituents as measured from
the CM frame.  This latter ("internal") ke is not included in the
aforementioned WE theorem. The theorem is simply (the integral of) F=ma
applied to the motion of the system CM.

Bob Sciamanda
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "RAUBER, JOEL" <JOEL_RAUBER@SDSTATE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, October 30, 2001 1:40 PM
Subject: Re: ENERGY WITH Q


> > 1) It's not true.  Consider a device with a "front wheel" and a "back
> > wheel".  Starting from rest, I spin up the wheels,
> > counter-rotating.  The
> > net torque around the CM is zero.  The motion of the CM is zero.  The
> > conjectured expression above is zero squared.  Yet the kinetic energy
> > increases.
>
> I think this is the best gedanken device yet that illustrates the point
John
> D is making.  Any comments from others regarding this example?
>
> Joel R.