Re: point particles

[Bob Sciamanda <trebor@VELOCITY.NET>]

If mg=qE the net force is zero, the CM remains at rest, and so you ARE
riding it  ;>)

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.velocity.net/~trebor/
trebor@velocity.net
----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, January 22, 2004 4:24 PM
Subject: Re: point particles


> > Isn't it so that (I think it's Chasle's theorem):
> > The instantaneous general motion of any rigid body can be viewed as the
> > compounding of
> > 1) a pure translational motion of the body, and
> > 2) a rotation about any freely choosable point fixed in the (extended)
body.
> > It follows that if one rides on a fixed body point and observes the body,
> > only a pure rotation will be directly observed at any instant.
> > I think this is quite intuitive.
>
> I would agree that what you have said here is not only quite
> intuitive, but, in fact, almost a tautology.  In the case under
> consideration, however, no "riding" is actually required.  I simply
> ask you to consider the motion of a rod with a UNIFORM mass
> distribution and positive electric charge at ONE end subject to
> oppositely directed and uniform gravitational and electric fields
> with mg = qE.
>
> John Mallinckrodt
> Cal Poly Pomona