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Loose rivets
7th Feb 2009, 08:12
There is a discussion running on another forum which involves pendulums. A clock pendulum to be precise.

On this one, there is a pulse applied when the swing gets too 'weak'.

The "Hipp Toggle" was used for over one-hundred years, and is rather clever, since it only allows top-up pulse if the swing is getting below range.

What I need to know is what effects cause the pendulum to slow down. I've guessed at mechanical friction and aerodynamic loads, even gravitational anomalies and perturbations. But all a bit vague...can anyone give a clearer reason for the loss of swing?

Noah Zark.
7th Feb 2009, 08:21
'Tis gravity. When the pendulum passes bottom dead centre, it is in effect, travelling uphill, away from the earth. Dear Mother Earth, realising the gravity of the situation, pulls it back. :ok:

Loose rivets
7th Feb 2009, 09:02
You forgot the resistance of the conveyor belt....and what's more, ow come you've got so many posts? They won't let me keep mine.

Flight_Idle
7th Feb 2009, 09:07
It is air resistance & friction which slow a pendulum, if it wasn't for those it would swing forever.

tony draper
7th Feb 2009, 09:12
If it were as simple as that surely a pendulum operating in a vacuum would tick forever? more like a oscillation damped by gravity one thinks.
:)

BladePilot
7th Feb 2009, 09:18
Chambers Street Museum in Edinburgh used to have a Pendulum thingy (I think it was called by some other name) which hung from the roof in the main entrance hall a huge big open space about four levels high.
A large silver ball with a needle protruding from the bottom was connected to a roof support beam by a very long fine steel cable. The thing was in constant motion apparently caused by the rotation of the Earth and nothing else, amazing. Unfortunatley it has since been removed and is kept in some dark basement storage area, pity.

tony draper
7th Feb 2009, 09:28
That be Corriolis force I think, the deflection to a large mass caused by the rotation of the Earth,Super Tankers had to take it into account when heading north or south,southerly they their course would drift to the right and visa versa,err it might be the other way round I int had mecoffee yet.
Dont think it effects thoses wee pipsqueak airyplanes you lot flit about in.
:)

Rainboe
7th Feb 2009, 09:58
They must have been doing something to give it an occasional 'twang' or it would slow down! Whilst gravity may damp down a pendulum on the upswing, don't forget it equally speeds it up on the downswing! All that damps down a pendulum's motion are:
Air resistance of movement
Pivot friction
Nothing else.

TRC
7th Feb 2009, 12:29
All that damps down a pendulum's motion are:
Air resistance of movement
Pivot friction
Nothing else.


Er, aren't we forgetting WHY a pendulum is fitted in a clock?

That'll slow it down won't it?

Lon More
7th Feb 2009, 12:35
Took death to stop my grandfather's clock

Flight_Idle
7th Feb 2009, 12:35
If it were as simple as that surely a pendulum operating in a vacuum would tick forever? more like a oscillation damped by gravity one thinks.

But if you increase the gravity, the pendulum swings faster!

tony draper
7th Feb 2009, 12:36
How the cuckoo knows what time it is to come out baffles me.:uhoh:

Bushfiva
7th Feb 2009, 12:43
You might want to consider the special case of a pendulum with a period of 84 minutes, and its link to IN systems.

Sallyann1234
7th Feb 2009, 13:05
If it were as simple as that surely a pendulum operating in a vacuum would tick forever? more like a oscillation damped by gravity one thinks.

No, Rainboe is right (bugger it!). The pendulum should retain a constant amount of energy which changes from all potential at each end of the swing to all kinetic at the bottom of the swing. But it slows down due to air resistance and pivot friction, which converts some of the energy to thermal. In a vacuum there is no air resistance, but the pivot friction is still there so it will eventually stop.

Rainboe
7th Feb 2009, 13:08
Gravity, neglecting other factors, both equally damps down and speeds up a pendulum every cycle. In a vacuum, neglecting pivot losses, indeed a pendulum would swing forever. What other factors acting on it are there?

Cornish Jack
7th Feb 2009, 13:13
Re. Foucault's pendulum, there is/was a working copy of the original in the Trianon in Paris. Only been to Paris once and went on a vague 'walkabout' and came to the Trianon by accident. Knew 'nowt' about it but as a child much of my reading was from the Children's Encyclopaedia and it featured a dark grainy picture of Foucault's Pendulum and the details of its indication of the Earth's rotation. Was utterly transfixed to see it 'in the flesh' - one of those magical days/occasions:D

tony draper
7th Feb 2009, 13:34
Re that song Mr More, I have seen that grandfather clock that stood 90 years on the shelf,tiz in a small Hotel down Whitby way, the Hotel manager was proudly telling me its story and got annoyed when I showed more interest in his Jack Russel Terrier.
:rolleyes:
Ah here it be,long time since I heard this song,tiz a true story you know,had that straight from the horses mouth.
YouTube - Grandfather's Clock (http://www.youtube.com/watch?v=f3tlIMJ9bK0&feature=related)

Miserlou
7th Feb 2009, 13:53
Chaos theory has some contribution to make also. Something along the lines of not taking as regular course as one imagines.

But the mechanical damping and aerodynamic resistance are the damping forces.

Davaar
7th Feb 2009, 14:49
Took death to stop my grandfather's clock

It wasn't exactly a clock, but during my apprenticeship I did the executry of an old chap's estate. Among the papers was a succession of bank pass-books, day by day, year by year, and the last entry on the last line marked the old gentleman's passing.

BladePilot
7th Feb 2009, 16:23
Was that a Pendulum in Linford Christie's shorts or was he just happy to be running:)

iws
7th Feb 2009, 19:29
Even when a pendulum eventually runs down, it can never hang still vertically 'cos of Heisenberg's Uncertainty Theory (Zero point energy).

Loose rivets
7th Feb 2009, 19:47
'twas all about this chap restoring a fine clock. He wanted to overcome the poor mechanism reliability by inputting a timed pulse. I argued that it would be "Pushing time into the pendulum."

The pulse and natural frequency of the pendulum would have to be perfectly in phase. Or a feedback would have to be contrived. But this was all a shame, cos the mechanism was a mainstay for 100 years and was more than a little clever.

It seems that it only got a boost pulse when the swing amplitude became too 'shallow'. Think about that one...quite difficult to do when they had only recently made electricity go round bends in a coil. Higgs died before the end of the last century.

Peter Fanelli
7th Feb 2009, 19:51
You might want to consider the special case of a pendulum with a period of 84 minutes


If only they only lasted 84 minutes, one could simply escape to the pub for a couple of hours.

iws
7th Feb 2009, 20:40
The trick to pulsing a pendulum is to set it very slightly slow. When it is pulsed it speeds up just enough to lock it to the pulse source.

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....).

Incidentally, it is relatively easy to see the effects of the Moon's and Sun's gravitational pulls on a free-running pendulum - about one part in 10 to the -5 at worst case.

tony draper
7th Feb 2009, 20:48
Harrisons later Chronometers did not have pendulums, from whence did Mr H's timepieces derive their pulse?.
:confused:

iws
7th Feb 2009, 20:50
Precison balance wheels on torsion springs.

Flash2001
7th Feb 2009, 21:41
As most pendulum bobs are made of metal, the earth's magnetic field will cause some eddy current damping. Pivot friction can be avoided by using a piece of spring material at the top of the suspension at the expense of adding another term to the equation.

After an excellent landing you can use the airplane again!

Noah Zark.
7th Feb 2009, 21:47
Loose Rivets
....and what's more, ow come you've got so many posts? They won't let me keep mine.........................they haven't let me keep mine either!

Davaar
7th Feb 2009, 22:40
Of course, they quite often come with a blade strapped to the foot that comes closer and closer to a chap underneath in a small room. He is not well up on the theory of the pendulum, and figures it will do him grievous bodily harm.

He edges away from the blade towards a pit in which lurks a nameless Horror, not in itself an attractive prospect.

He might take the other direction or directions, but that/those way/s the walls are of red-hot metal. You might say he is on the horns of s dilemma.

I always avoid two things: (a) pendulums and (b) blokes by name of Poe.

tony draper
7th Feb 2009, 22:57
Did you know that E A Poe was a rabid Englandophile and considered that the War of Independence was the most appalling mistake ever made by anybody in the entire history of the world.
True that is.
:E

Loose rivets
7th Feb 2009, 23:05
He could well have been right!



As for being in the pit of horrors, Edgar could have escaped in a moment...If he'd used an Allen key.:}


Oh, alright, an Allan key, if you want to be pedantic.

Flight_Idle
7th Feb 2009, 23:31
Pivot friction can be avoided by using a piece of spring material at the top of the suspension at the expense of adding another term to the equation.

Nope, spring material will not negate friction!

Davaar
7th Feb 2009, 23:57
Oh, alright, an Allan key, if you want to be pedantic.

I don't suppose an Edgar key would have done at a pinch?

airfoilmod
7th Feb 2009, 23:59
Ho key Poe key

Cpt_Pugwash
8th Feb 2009, 00:04
Quote:
You might want to consider the special case of a pendulum with a period of 84 minutes


Bushfiva,
You have clearly encountered Schuler tuning/damping at some time.:)

Rollingthunder
8th Feb 2009, 00:42
All that damps down a pendulum's motion are:
Air resistance of movement
Pivot friction
Nothing else.

Except for one of my little fingers.

They have one here in a bank downtown. It's about 80 feet long.
Mounted horizontally, equal lengths from the pivot point, each end has large openings like jet intakes.
It makes quite an amazing sound.

Flight_Idle
8th Feb 2009, 00:46
Is that something to do with the pendulum appearing to be stationary as the Earth rotates?

Flash2001
8th Feb 2009, 01:54
FYF FI

If there is no pivot there is no pivot friction. A short length of flat spring can be substituted for the pivot.

After an excellent landing you can use the airplane again!

Loose rivets
8th Feb 2009, 04:39
Oooh no! Atomic forces in the metal and all that.




I don't suppose an Edgar key would have done at a pinch?


He tried that...it didn't work, and left him Poe-faced:hmm:

Sallyann1234
8th Feb 2009, 11:27
If there is no pivot there is no pivot friction. A short length of flat spring can be substituted for the pivot.

No. Bending the spring requires energy.

mixture
8th Feb 2009, 11:40
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 ?

Tone
8th Feb 2009, 13:03
Just read in the Times that the caesium clock is about to be replaced by a laser 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!

iws
8th Feb 2009, 13:34
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.

Tyres O'Flaherty
8th Feb 2009, 13:48
The original article ( I believe ) is actually in New Scientist, which I read a few days ago.

The laser 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

matt_hooks
8th Feb 2009, 14:12
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.)

Flash2001
8th Feb 2009, 15:04
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!

Lon More
8th Feb 2009, 15:36
I suppose the drag would slow it down a bit:E
http://www.lileks.com/bleats/archive/07/1007/1005art/poster1.jpg

RJM
8th Feb 2009, 17:57
Shh, Loose Rivets, you know what happened to Richard Taylor! :uhoh:

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.

Loose rivets
8th Feb 2009, 18:04
I say naaaaaaaaaaaaaathing. :oh:

mixture
8th Feb 2009, 18:57
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 ?

matt_hooks
8th Feb 2009, 19:19
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.

iws
8th Feb 2009, 21:10
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.

iws
8th Feb 2009, 21:16
One thing I forgot - you would, however, need to know your precise location!

Flash2001
8th Feb 2009, 21:17
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!