Re: Lecture Notes

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

----- Original Message -----
From: "Jim Green" <JMGreen@SISNA.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, June 29, 2000 4:33 PM
Subject: Lecture Notes


> This morning I was surfing about and came across the following taken from
a
> lecture note page. I am not comfortable with this wording. Comments???
>
> To understand the tide-generating force you need to understand centripetal
> force. Centripetal force that keeps the Earth-moon system together is
provided
> by gravity.

Well, gravity certainly supplies the centripetal force that keeps the
Earth-moon system together as they revolve around their common CM.

> All particles of the Earth follow circles of equal radius as they rotate
in the
> Earth-moon system.

This is not correct. Earth particles move in circular paths as the Earth
rotates, but their motion in space is more complex as the rotating Earth
revolves about the Earth-moon CM.

> The centripetal force required to keep each particle in an identical orbit
is the
> same and is directed toward the center of the orbit.

This is true in the restricted case of particles in identical orbits. But
that includes only particles having a common latitude and a common distance
from Earth's center.

Paul O. Johnson