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Re: A weighty subject



Richard,
In part b) the resultant of the scale force and F_g must provide a
centripetal force on m!

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Richard Tarara <rtarara@SAINTMARYS.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, October 15, 1999 11:43 AM
Subject: Re: A weighty subject


----- Original Message -----
From: John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>

I've proposed exercises that I thought might reveal some conceptual
difficulties previously in this thread. Nobody took me up on them, so
maybe I shouldn't bother trying to do it again. Nevertheless,
undaunted
...

Consider two identical, spherically symmetric, nonrotating planets of
radius R and mass M orbiting their common center of mass in a circle
of
radius 2R (and don't worry about Roche limits and all that!)

a) What is the Newtonian "force of gravity" on an object of mass m
near
the surface of one planet when the other is directly overhead.

8/9(GMm/R^2) from GMm/R^2 - GMn/(3R)^2


b) What would a properly functioning scale read if it were used to
weigh the object?


8/9(GMm/R^2)

c) If the object were allowed to fall, what would be its acceleration
relative to the nearby planet?


8/9(GM/R^2) (approximately over a short range near the surface)


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


I would say all in agreement with W = m x (Grav Field). What am I
missing?
The second planet certainly affects the net gravitational force, the
field,
and hence the weight and acceleration.

Rick