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Re: point particles



John, I share your pain!
Your intuition is thwarted by "the magic of rigid body forces". These
forces cause the far (uncharged) end of the rod to pivot so as to keep the
CM stationary.

I was long bothered by the magic of rigidity. EG: how does a force get
multiplied across the fulcrum of a lever?

I finally quieted my intuition by using Interactive Physics to model a lever
with a system of masses and springs (monitoring the spring/applied forces
on each mass) in a myriad of situations. The results, evolving in time
through damped oscillations, directly show the evolution of these forces and
the subsequent motion of the (semi-rigid) object.

Since I retired, I no longer have access to IP (the school's). Now you have
so peaked my interest that I am going to order my own copy of IP and play
further! Go thou and do likewise (play) : > )

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 5:22 PM
Subject: Re: point particles


I wrote:

In the case under consideration ... 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.

To which Bob wrote:

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

True enough and perhaps sufficient "explanation" for those for whom
the mathematics IS intuition.

But as a final attempt to put perhaps too fine a point on it,
consider the rod I described held horizontally in front of you. Now
simply let it go. What does your intuition tell you it should do?

I can explain why it does what it does in terms of Newton's laws, but
it still really bothers me!

--
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm