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Pendulums and things that stop them

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Pendulums and things that stop them

Old 8th Feb 2009, 10:40
  #41 (permalink)  
 
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I have a Cesium Beam frequency standard from which I get 1 second pulses that I feed to one of my pendulum clocks (just for fun you understand....).

Perhaps you could explain something that I've yet to get my head around properly.

I know that a second is defined based upon transitions of Cesium levels.

However what I don't "get", is how you go from there to knowing what the time is ?

It's all very well watching the exact seconds tick by, but how do you decide when it's exactly 11:40 GMT ?
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Old 8th Feb 2009, 12:03
  #42 (permalink)  
 
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Just read in the Times that the caesium clock is about to be replaced by a [email protected] clock which will be accurate to within one second in 1.7 billion years. As a result it will be possible in the future to build "an autopilot accurate enough to land a plane without human intervention"

Gosh!
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Old 8th Feb 2009, 12:34
  #43 (permalink)  
iws
 
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It all depends on what you mean by what the time is.

If you are thinking of clock time, that is locked to the Earth's rotation, so that the Sun rises at the correct times every year, and transits of stars measured by telescopes are still used for calibration.

Unfortunately, the Earth is slowing down due to tidal friction from the Moon and Sun, so Earth time has to be adjusted every so often to keep it in sync with the more accurate Atomic (Cesium) time. This is done by inserting "Leap Seconds" into the Earth (UTC or GMT) time.
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Old 8th Feb 2009, 12:48
  #44 (permalink)  
 
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Tone

The original article ( I believe ) is actually in New Scientist, which I read a few days ago.

The [email protected] clock will eventually be accurate beyond the present life of the Universe, i.e. losing less than a second in the generally accepted 13.7 billion yr


(edit)

The level of accuracy is such that it will allow Sub-meter GPS measurement. There are some mooted problems with installing some of the necessary optical eq.pt on the sats but these are not insurmountable
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Old 8th Feb 2009, 13:12
  #45 (permalink)  
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The spring steel is a nice idea, except that you then run into hysteresis, some of the energy from the pendulum goes into heating the metal up and is "lost"

A well made ruby bearing will lose far less energy than a spring pivot. (and this is, indeed, the origin of the "jewels" in a watch. Originally it referred to the number of jewelled pivots.)
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Old 8th Feb 2009, 14:04
  #46 (permalink)  
 
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Aright aready!

A good spring is pretty close to perfectly elastic, it returns most of the energy stored in it. Better than any form of bearing I can think of. WTH is a spring pivot? That's not what I proposed.

After an excellent landing you can use the airplane again!
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Old 8th Feb 2009, 14:36
  #47 (permalink)  

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I suppose the drag would slow it down a bit
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Old 8th Feb 2009, 16:57
  #48 (permalink)  
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Shh, Loose Rivets, you know what happened to Richard Taylor!

Surely gravity has a lot to do with it? Substitute a magenet for gravity and swing a metal ball on a string above a magnet. Eventually the magnet will stop the pendulum in its attempt to pull the ball towards it.
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Old 8th Feb 2009, 17:04
  #49 (permalink)  
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I say naaaaaaaaaaaaaathing.
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Old 8th Feb 2009, 17:57
  #50 (permalink)  
 
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iws,

Yes I was referring to clock time.

Basically, if I were to set up shop in the middle of nowhere (e.g Australian outback), what would I need in addition to one of your fancy cesium frequency standards ?

i.e. on the basis of zero external input (no phones, faxes, internet) .... how do I decide what the clock time is ?

I know the old story of the sundials, but that seems too inaccurate... are you really saying watching the stars is more accurate ?
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Old 8th Feb 2009, 18:19
  #51 (permalink)  
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Mixture. Given a decent set of tables, the sky is a remarkably accurate way of determining local time.

And yes, the "pivot" bit was a typo. I still reckon a good jewelled bearing would give you about the minimum possible loss of energy at the hanging point.

Rjm, the gravity, or indeed the magnetism, without some other kind of force to remove the energy from the system, will not cause the system to slow down. All of the energy that the pendulum has at the top of the swing, in the form of potential energy, is transferred to kinetic energy by the bottom of the swing. As the pendulum rises again, the energy is transferred back into potential energy. The total energy is "conserved" unless there is some kind of friction or other method of removing energy from the system.
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Old 8th Feb 2009, 20:10
  #52 (permalink)  
iws
 
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As Matt suggested, yes the sky is the final arbiter as regards clock time, because clock time is based on the rotation of the Earth. Essentially clock time requires the Sun to rise and set as close as possible to the same times every year. There are professional Observatories whoes job it is to check the star transits precisely, but any Observatory looking at the stars
can work out the Earth's clock time and compare it with their Atomic clocks, because the measurements of telescope position are so good these days.

If you were in the outback nowadays, you would use GPS. This is locked to Atomic time, but most receivers can read the correction of leap seconds which is broadcast in the GPS signal, and correct to read UTC (Clock time) as necessary.
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Old 8th Feb 2009, 20:16
  #53 (permalink)  
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One thing I forgot - you would, however, need to know your precise location!
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Old 8th Feb 2009, 20:17
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I guess that steel on sapphire is maybe 2 or 3 times better (C of F) than steel on brass. that's nowhere near as good as the coefficient of restitution of a good spring. Think tuning fork say. I think that you'll find that the jewelled bearing system was employed because of it's superior wear characteristics and constant behaviour under temperature change.

After an excellent landing you can use the airplane again!
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