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Re: cavendish expt



This sounds cool! I'll have to try it! Are you using a laser & mirror to
detect the torque? The torque is so small that it may not be visible.
Bouncing the light off of the mirror (as Cavindish did) magnifies the small
motion. The flat cross-section of the videotape is a potential source of
unwanted torque, although I have no feel for how much this would affect the
experiment. My first inclination would be to try the thinnest nylon
monofilament possible, or perhaps some braided Kevlar fishing line. Yes, air
currents will be a problem, including those created by the experimenter's
adjusting of buckets. It may be possible to look for changes in the simple
harmonic motion of the torsion pendulum, rather than just a static torque
due to the buckets. Please let us know how this apparatus works out!

Vickie Frohne

-----Original Message-----
From: Wolfgang Rueckner [mailto:rueckner@FAS.HARVARD.EDU]
Sent: Tuesday, November 12, 2002 11:13 AM
To: PHYS-L@lists.nau.edu
Subject: Re: cavendish expt


Greetings phys-eders:

I am attempting to set up an experiment similar to
what Cavendish did when he "weighed the earth".

I'm using two joint compound buckets filled with sand
as the larger masses. I have a meterstick horizontally
oriented with bottles of sand on each end. This is
suspended from the ceiling with video tape.
|
|
/\
/ \
/ \
________
0 0

OO OO
(In front) (In back)

I'm finding that it takes an incredibly long time for
the suspended meterstick to find its equilibrium
position. Once it does, I'll move the buckets in
closer to the bottles of sand and see if I can detect
any torque on the meterstick.

My questions are:

1)Do you think the incredibly small Fg would produce
any effect?

2)Have any of you tried this before? If you did, how
did it work?

3) Any suggestions?

Two suggestions: Instead of buckets, maybe try 55 gal drums filled
with water to maximize the effect? Also, you may have to shield the
torsion apparatus from air currents in the room -- maybe a sheet of
plastic (painter's drop cloth) shaped into a cylinder-like
surrounding? Wolfgang

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