Re: [Phys-l] Q from amplitude width?

[Bernard Cleyet <bernardcleyet@redshift.com>]

On 2011, Apr 26, , at 09:49, brian whatcott wrote:

> This then provides a figure for the actual power required to maintain 
> that amplitude for that bob weight.
>
> If the losses were all in the aero drag, then the maintaining power 
> ought to be rather similar for various masses.  

It's the water drag, but only a "detail".

>   This is not the case 
> for your numbers.

You mean the C values in the fit equation?  Yes, but my app. is very crude.  I'm more surprised they vary so little!   ~ 9.4, 8.4, and 6.1

Furthermore, the plot of Q(mass) is "too" good!   The water drag masks the increased support and suspension loss, which is very evident without the "artificial" water drag.


> One could assign this increased power requirement at 
> increased mass  to increased aero drag (with mass) and/or increased 
> suspension loss (with mass)

The increase from 331gm to 976 is only 11%, so I suspect it's apparatus and bc technique (patience)** failure, not increased drag, suspension and support loss.  [Is the % difference between the average a better measure?]

> Brian W
> _

Evidently, I changed the URL? after posting it.  Here the correct one, I pray.

http://www.cleyet.org/Pendula,%20Horological%20and%20Otherwise/Pendulum%20response%20to%20E-M%20harmonic%20drive%20as%20a%20function%20of%20bob%20mass/Pendulum%20response%20to%20E-M%20harmonic%20drive.tiff


** My new Pasco digital function generator-amplifier will require less patience.  Its display agrees rather well w/ my TCXO HP counter, also.   (Thanks to RAFT)


bc appreciates JD and BW's comments.

--------------------------------

The almost complete post:



On 4/25/2011 11:34 PM, Bernard Cleyet wrote:
> At 100Hz my LCR SRS meter displays ~ 200mH and Q ~ 0.3  at 10kHz the Q 
> rises to ~ 30, so I suppose, as  any coil will have a resonance from 
> the distributed capacitance.  (assuming the wire is copper and not 
> nichrome!)
>
> ~ 5.5 / 2Pi ~  0.88Hz.
>
>
> If you go to:


/snip/

Going back to basics, it's interesting to assign an equivalent length to 
your pendulum, and supposing the mass is all in the bobs, assign a 
potential energy for the position where the bob reverses direction.
Your values for Q then provide a measure of the potential energy lost 
(as height) for each bob weight per cycle of free decay.

This then provides a figure for the actual power required to maintain 
that amplitude for that bob weight.

If the losses were all in the aero drag, then the maintaining power 
ought to be rather similar for various masses.    This is not the case 
for your numbers. One could assign this increased power requirement at 
increased mass  to increased aero drag (with mass) and/or increased 
suspension loss (with mass)

Brian W