Re: Lecture Notes

["Paul O. Johnson" <pojhome@SWBELL.NET>]

----- Original Message -----
From: "Jim Green" <JMGreen@SISNA.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Saturday, July 01, 2000 12:18 PM
Subject: Re: Lecture Notes


> Paul, this is a lady professor -- and I don't think that she is as nearly
correct as
> you give her credit for:  She seems to be asserting that all the particles
in the
> Earth are moving about the Earth/Moon cm _in the same orbit_  and
(assuming
> the same mass for each particle I take it) have the same force acting on
them.
> Now it is true that I am old and senile, but that can't possibly be
correct -- even
> if the bodies don't spin on their own axes,  Did I miss something here?
>
> Jim

Well, if that's what she's saying, Jim, then she is flat wrong. I don't see
that you are missing anything.

Even if she ignores the Earth's rotation, the particles comprising Earth and
moon move about the Earth-moon axis of revolution in many different orbits.
Note, also, that they are all revolving about a line (the axis), not about a
point (the CM).

Only those particles that are the same distance from the axis of revolution,
and that lie in the same plane normal to the axis, move in the same orbit.
Since the Earth is (for this purpose) a rigid solid (not including the
oceans, of course), such particles would also have the same tangential speed
and would therefore experience the same centripetal force.

We codgers have to stick together. Your doubts are not misleading you. Tell
her, nicely of course, that she's off base.

Paul O. Johnson