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Re: [Phys-l] Another problem Was: Re: real-world mechanics problem



?A first shot:
It seems clear to me that a change in g would certainly directly affect the omega and T values.
But it also seems that just how the amplitude is afffected by the g variation would depend on the details of the driving mechanism.

-----Original Message----- From: Bernard Cleyet
Sent: Sunday, February 06, 2011 1:14 PM
To: Forum for Physics Educators
Cc: Bryan Mumford ; Nancy Seese
Subject: [Phys-l] Another problem Was: Re: real-world mechanics problem

OML!

In checking the total E derivation (example in The Berkeley Physics Course V 1), I missed the use of the approximation!!

So both methods use the approximation. Sorry for all the bother.


Now here's another problem:


What happens to the amplitude when g in T~= 2Pi (l/g)^0.5 is changed?

From the total E derivation (after the approximation):

Angular speed (at BDC) = A(g/L)^0.5 or

(speed/A) / L^0.5 = g^0.5

so if g is reduced the ratio becomes smaller. Which changes or both?

This is not just a theory problem, but practical.

V. good pendula detect the tidal change of g due to the moon. One clock, the Littlemore**, thought to be the best made, did a poor job of detection compared to a Shortt clock***. Bryan**** thinks it's due to its method of maintaining constant amplitude (to prevent circular error). His description of his moon tidal model and a discrepant amplitude measurement is here.****

To model tidal effect, Bryan added a repulsive force on the bob using an electromagnet and a PM on the bottom of the bob. Unfortunately, this has the complications of, at least, non-homogenous field and eddy current loss. I think the orientation of the magnet WRT the field also changes.

I propose repeating the experiment using a free pendulum***** w/a non conducting bob and rod, a ceramic magnet, and Helmholtz coils. Before I go to all this trouble, I'd appreciate v. much comment from you-all, especially on the effect of g change on the amplitude.




** http://www.hsn161.com/HSN/hsn_article.php

*** http://en.wikipedia.org/wiki/Shortt-Synchronome_clock

**** http://www.cleyet.org/Someone_is_Wrong/The%20Littlemore%20Clock/The%20Littlemore%20clock%202.pdf

***** My rotary motion sensor suspension, half meter rod and 150gm bob pendulum that I used for previous experiments will do well, as its Q is ~ 1k.

bc worships Lucifer, the bearer of light.

On 2011, Feb 06, , at 01:32, Bob Sciamanda wrote:

?BC,
Then you have answered your problem as stated in your original post:

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Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/




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