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Re: impulse/momentum



It was an horological observation of this kind (that a single pendulum reacts
on its support) concerning the sources of timing error, that led to twin
pendulums (pendula?) supported on a single beam.
Because the reaction was reacted much closer to the source - seasonal,
atmospheric pressure and temperature variability could be improved.

It is a short step from this development:- contra-beating pendulums) to
using
glorified tuning forks. Then finding flex modes of quartz which could provide
relatively low timing frequencies in a compact package along with miniature
discrete then integrated electronics, exported the splendid temperature
stability of fused quartz into the personal timepiece from the observatory.

Though molecular beams have been resistant to miniaturization, an alternative
approach is slaving a watch to a molecular master. There are examples of
this
approach - and in wall clock form, the technology comes amazingly cheap.
The last module I bought was under $15 as I recall.
The effective slaving range is about 2000 miles, so the approach is not
pan-potent if Low Frequency broadcast masters at Colorado are used.

If one goes to a GPS reference, then world-wide coverage is available, and
is now often used in time-critical applications. Here, the variability comes
from the background time code export time delay at the user location

Brian W

At 12:54 AM 12/14/2003, you wrote:
"Yesterday someone asked
in class what happens to conservation of momentum when you have a pendulum
swinging. I find it easier to consider first a rigid earth wobbling back
and forth with the pendulum. The pedagogical point is that there must be at
least two bodies interacting before there can be a pendulum. Realistic
modelling of the earth can be added as a footnote."


One of my more favoured demos. is the pocket watch on a platform hanging
by three threads suspended by a single one. Glued (tacky wax) to the
watch is a mirror. A pocket diode laser's beam reflecting off the
mirror and onto a, not so nearby, wall clearly shews the watch's
"wobbling".

bc

Mark Sylvester wrote:

>Actually, I was just indulging in a little pun over breakfast coffee...
>
>...but in my experience, far from faltering, the juvenile mind thoroughly
>enjoys following through the logic that leads to the conclusion that "the
>earth moves" when I jump into the air. And this is not an article of faith.
>It's the logical exploration of a model. In this case the model requires a
>perfectly rigid earth, so it's not realistic, but physics is full of such
>creatures and it's often useful to talk about them. Yesterday someone asked
>in class what happens to conservation of momentum when you have a pendulum
>swinging. I find it easier to consider first a rigid earth wobbling back
>and forth with the pendulum. The pedagogical point is that there must be at
>least two bodies interacting before there can be a pendulum. Realistic
>modelling of the earth can be added as a footnote.
>
>Mark
>
>PS looking back over the thread, I see Justin Parke asks "Any suggestions
>
>
cut


Brian Whatcott Altus OK Eureka!