Re: [Phys-l] Another practical problem

[Bernard Cleyet <bernardcleyet@redshift.com>]

On 2011, Feb 15, , at 17:30, brian whatcott wrote:

> Ho hum... a thought experiment:-   machine three bobs as right cylinders of
>  constant diameter:  one of aluminum, one of lead: both these of the
> same mass: and a third bob, this one of the same diameter and volume
>  as the aluminum  one, but made of lead.

cut

> Conclusion: if your experiment did not confirm that shedding bobweight
>  reduces Q and hence reduces steady state  amplitude for constant 
> energy input,
> you did not control enough factors    :-)
>
> Brian W



Brian!

Not quite my problem.   I'm not using a clock w/ a "driving" escapement.*  I'm trying to explain the ("instantaneous")** change in amplitude when a portion of the bob falls from a freely decaying pendulum.  I do find changes in the Q, because I fit to an exponential decaying amplitude to find the amplitude.  [decay constant  ~ 0.003)  This "study" was prompted by my friend's failure to detect a change in amplitude (measured by the flag interrupt time at BDC of a photogate) when "sliding off" three quarters (0.02kg) from a 77 kg tower clock's bob. *** [Easily explained by the minuscule mass (0.02kg) compared to the bob's, and the detector's insensitivity.]  I find significant changes in amplitude using a rotary motion detector as the suspension and electromagnetically dropping a 0.07kg mass from a 0.30kg pendulum (rod 0.025kg).
---------------------
pics here:

http://www.cleyet.org/Horological/weight%20pan%20effect/

I've just increased the mass dropped for further trials from 0.07 to 0.120kg.
 ---------------------
Your discussion is well known to horologists (centuries), and is complicated by support loss.  A few years ago an horologist (AH) did a study exactly as you describe w/ constant size using W, brass, steel and Al. (IIRC)  The expected Q increase w/mass did not occur, because of the increased support loss even when the support (many kg cast iron) was bolted to a concrete column.  In the clock's wood case the clock failed to operate w/ the W bob!   He measured the motion of the support, inter alia, using a light lever.  

I reported this back in '07:

https://carnot.physics.buffalo.edu/archives/2007/06_2007/msg00024.html

n.b. my confusion of force and E.


* Not incidentally, the better clocks vary the drive to maintain constant amplitude as part of the method of mitigating circular error.  e.g. the Synchronome and Littlemore.  I don't remember what Woodward did w/his clocks.

** Now I understand the cause of the confusion.  I shoulda writ originally "instantaneous change" not implied a change in the change (decay) of the amplitude when part of the bob drops.

*** there's another problem here.  If the photogate did detect a change in amplitude, it would not be correct unless the mass' original position was above the C of M of the bob.  Otherwise, there would be an unintended torque change and the position of BDC would change. I position my added mass on the electromagnet so that BDC doesn't change.

bc

p.s.  Indeed, the decay constant does increase for the 0.12kg drop from  0.00239 to 0.00278   Using the usual assumptions (i.e. linear dissipation and decay constant << 1)  Q = angular frequency / 2(decay constant), my result is: before drop, Q= 1076 and after, Q = 944