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Re: point particles



"I would agree that what you have said here is not only quite intuitive,
but, in fact, almost a tautology."

Goldstein writes of Chasle's theorem: "Detailed proof is hardly
necessary."

Often the more obvious (intuitive) theorems require very subtle proofs
that the picayune demand.

bc, who notes Poinsot's theorem (corollary?) also applies.

John Mallinckrodt wrote:

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