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Pendulums and things that stop them

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Pendulums and things that stop them

Old 7th Feb 2009, 07:12
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Pendulums and things that stop them

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?
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Old 7th Feb 2009, 07:21
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'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.
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Old 7th Feb 2009, 08:02
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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.
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Old 7th Feb 2009, 08:07
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It is air resistance & friction which slow a pendulum, if it wasn't for those it would swing forever.
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Old 7th Feb 2009, 08:12
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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.
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Old 7th Feb 2009, 08:18
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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.
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Old 7th Feb 2009, 08:28
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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.
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Old 7th Feb 2009, 08:58
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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.
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Old 7th Feb 2009, 11:29
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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?
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Old 7th Feb 2009, 11:35
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Took death to stop my grandfather's clock
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Old 7th Feb 2009, 11:35
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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!
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Old 7th Feb 2009, 11:36
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How the cuckoo knows what time it is to come out baffles me.
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Old 7th Feb 2009, 11:43
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You might want to consider the special case of a pendulum with a period of 84 minutes, and its link to IN systems.
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Old 7th Feb 2009, 12:05
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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.
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Old 7th Feb 2009, 12:08
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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?
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Old 7th Feb 2009, 12:13
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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
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Old 7th Feb 2009, 12:34
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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.

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
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Old 7th Feb 2009, 12:53
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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.
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Old 7th Feb 2009, 13:49
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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.
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Old 7th Feb 2009, 15:23
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Was that a Pendulum in Linford Christie's shorts or was he just happy to be running
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