Foucault's Pendulum Recreation for Physics Project

[Brad Meacham]

(I apologise if this is the wrong area to post)
Hello everybody

I am planning on building my first Focault Pendulum(As a physics projject for school) and I have a few questions. I am going to purchase a cable(it needs to be smooth and friction-less around 7 feet), I also am going to need to purchase a bob (around 2-3lbs). Now my first question is where can i buy these items? Online has only a few varieties and I do not like my options.

Also what would be the best way to suspend this cable from my ceiling in order to have it rotate 360 degrees without friction causing it to slow?

I was also thinking of purchasing a donut magnet and installing it at the top as a kicker (possibly with an iron collar); if there is too much friction/wind resistance and my pendulum would stop after 2 hours.

Any help, advice, questions, are all appreciated thank you very much for taking the time!

Brad

 

[Vanadium 50]

Also what would be the best way to suspend this cable from my ceiling in order to have it rotate 360 degrees without friction causing it to slow?

And that is the key question in your project.

 

[Andy Resnick]

I am planning on building my first Focault Pendulum(As a physics projject for school) and I have a few questions.

This is not a small-scale project, it requires careful attention to a lot of details. Here's a document with some specifics :

http://www.astro.louisville.edu/foucault/pendulum.pdf

 

[Brad Meacham]

I am not trying to go crazy I figure I would need a massless suspension/aircraft cable, a frictionless pivot, and a hollow bob. I am going to head over to the engineering lab next week so that I can work out some numbers. Is there a website online I can order from? or possibly a place around Portland, Or? Thanks!

 

[Nidum]

Google Images 'Plumb bob' and 'Plumb line' .

You can buy many varieties cheaply in tool stores and online .

 

[Andy Resnick]

I am not trying to go crazy I figure I would need a massless suspension/aircraft cable, a frictionless pivot, and a hollow bob. I am going to head over to the engineering lab next week so that I can work out some numbers. Is there a website online I can order from? or possibly a place around Portland, Or? Thanks!

If you find a source for massless cables and frictionless pivots, let me know- I would be interested in having some of those as well.

 

[jbriggs444]

If you find a source for massless cables and frictionless pivots, let me know- I would be interested in having some of those as well.

https://missionbelt.com/products/unobtainium (tongue deeply embedded in cheek)

 

[Brad Meacham]

what would you suggest I use as alternative materials?

 

[Brad Meacham]

http://www.e-rigging.com/cable
https://www.amazon.com/dp/B0009WG4U4/?tag=pfamazon01-20

How does this cable and plum look? I am working on finding a frictionless swivel they are tough to locate.

Thank you!
Brad

 

[Tom.G]

frictionless swivel they are tough to locate.

Closest I can come up with is a bar balanced on a knife edge, or a ball held up with a stream of air (but that tends to be unstable).

need a massless suspension/aircraft cable

Kevlar fiber (or thread). Light weight, super strong, a little hard to find, used in bulletproof vests.

 

[Brad Meacham]

Does anyone have a recommendation for another bob? or what about a swivel that works pretty well it does not have to be perfect.

 

[Tom.G]

http://www.e-rigging.com/cable
https://www.amazon.com/Johnson-Leve...i&ie=UTF8&qid=1477718638&sr=1-18&keywords=bob

How does this cable and plum look? I am working on finding a frictionless swivel they are tough to locate.

If you want a stiff cable that stuff should fill the bill. You may not be able to straighten it out though.

I think you need a much heavier weight so it has much more kinetic energy; air resistance you know.

 

[Brad Meacham]

What should I type into google to find a heavy spherical symmetrical weight I have tried every combo of words. I found this plumbob 32oz by luck...
Tom your information is incredible I know how to straighten cable so I was thinking of doing this:

get a large ball bearing, ideally plastic or unhardened (or at least not through hardened) steel and then drill and tap it for an attachment point for the pendulum. Then mount this plate with a hole or something like 95% of the diameter of the bearing, for this slip on pipe flanges are convenient. To get the best possible bearing surface I can lap the ball into the join with valve grinding paste or similar. This is a really advanced version, looks doable:

 

[John Mi]

Hi Brad, interested to hear how you went? Especially in how you implemented the 'kicker'.

 

[lesaid]

Hi Brad, interested to hear how you went? Especially in how you implemented the 'kicker'.

Hi - was interested too. I am attempting to develop a 'small' Foucault pendulum of my own - just for the challenge since everywhere says it can't be done on a small scale! For pivot - I am trying a device shaped like a question mark, pivoting on a point at the end of the 'hook', and a rigid pendulum (about 1.3 metres) attached to the downward shaft.

The kicker is a shallow, cylindrical electrode centred under the bob with about 10 kV from a high impedance power supply on it. Each time the bob passes (with a clearance of about 2 mm), a spark jumps which puts a charge on the bob and (briefly) repels the bob from the electrode until the charge dissipates. On the return swing, there is less charge left until the next spark. Amplitude of swing is about 1 cm either side of centre (easy to get larger amplitudes by increasing the voltage somewhat, but I think lower swing angles should be more accurate).

Bob is a clock weight, about 1.5 kg.

Running tests on it now (literally) - gradually finding preferred directions of swing and removing the causes.

Difficult to read angle of swing with any precision with this design - if it proves to work (precess as it should), then I'll think about reading angles more easily! Until then, I take video clips of the swing and can work out the behaviour from those after the experiment is done.

Might give you (or other experimenters in this) a couple of ideas. Interested in anyone with ideas to improve my design too.

 

[Vanadium 50]

Aren't your electrodes rotating with the earth? As you describe it, it sounds like you won't see the rotation of the plane of motion because it's effectively constrained.

 

[lesaid]

Aren't your electrodes rotating with the earth? As you describe it, it sounds like you won't see the rotation of the plane of motion because it's effectively constrained.

I don't think it should. I only have one electrode, which is cylindrically symmetrical, directly underneath the dead-centre of the swing - i.e. where the bob rests when it isn't swinging. The bob also is cylindrically symmetrical. So the impulse given by the electrode, random variability excepting, should be strictly radial, with no tangential component at all. So I think it 'shouldn't' (in a perfect world) have any effect at all on the direction of swing. What I'm trying to do now, as I adjust things, is get the world sufficiently 'perfect' that the rotation of the Earth is the dominant factor affecting the direction of swing.

 

[jfmcghee]

Not sure if you've seen this webpage: http://www3.sympatico.ca/surfin.dude/creative/clocks/foucault/foucault.html
His project is unfinished, but there are some ideas there. Among the links in the page is an old paper for the design of a desktop version. Still a dream of mine to build one for my classroom. Another drop in the bucket of dreams for classroom demos...

 

[lesaid]

Interesting links!

My pendulum is progressing slowly as I identify perturbing influences and modify the design to reduce them sufficiently. Some surprises along the way, and some mathematical challenges to understand the behaviour. I'm trying to do this in the simplest possible way with no electronics (other than a 10 kV power supply), no build techniques more sophisticated than hacksawing brass and soldering brass pieces together with a miniature blowtorch, and mostly made with what I've got lying around. Not, I'm afraid, a prototype for a classroom - more a 'home project' for fun, and hopefully, to prove that a small Foucault pendulum can be built by the ordinary DIY-er. But I guess, if it works, a free-standing desktop version could be developed.

Currently building a simulation model of the design to try to reproduce the behaviour I'm observing, and figure just how precisely my design needs to be built in order to be able to complete a full 24 hour rotation convincingly. So far, it has always settled to a preferred direction, or has oscillated between two perpendicular swing directions - but the period of that oscillation is reassuringly long - something like 90 minutes and I think I've identified a couple of reasons for it which I'm addressing now (very slightly different moments of inertia in different directions, and possible slight flexing of the support for the pivot in one direction).

I have some mathematical challenges with it too, as I try to model the behaviour analytically rather than by simulation.

Might be interesting to discuss this more if anyone is interested - in which case a new thread might be better than hijacking this one!

 

[lesaid]

Among the links in the page is an old paper for the design of a desktop version. Still a dream of mine to build one for my classroom. Another drop in the bucket of dreams for classroom demos...

You don't need such a sophisticated setup for a desktop Foucault pendulum. While I haven't sorted out the drive properly in mine, it now works as a free-swinging pendulum, and will show up to six hours (just) of precession, at as near as I can judge by eye on a marked piece of card, exactly the rate expected for my latitude. It has a 1.2 metre rigid shaft, a 1.4 kg bob and the pendulum sits on a point, supported by a small platform. The point is tipped by a 1 mm diameter ruby probe on a piece of glazed dinner plate as a hard platform. The pivot support structure is made partly out of wood, and partly out of pieces of brass bar soldered together.

Could build it in a day, given the materials - much simpler than the link that you posted.

I'm now working on quantifying the accuracy of the precession, and on the drive to get it to keep going indefinitely rather than just for a few hours.

Might be a viable project for your classroom. It needs great care to keep the moment of inertia independent of swing angle, and to have a rigid pivot support that cannot be significantly moved or twisted (even by a few microns) by lateral forces from the pendulum. Then it works! My test for these criteria is to start the swing in a circle. If the swing doesn't stay circular as it decays, then the system isn't symmetrical enough.

By the way - I start the swing off simply by releasing it from my fingers - not by burning threads. I'm struggling to understand why most people seem to find unwanted lateral movement such a problem. It is true that in the last stages of the swing decay, the swing becomes circular, but by that time the swing amplitude is maybe 5% of what it started with - not a problem in practice.

 

[sophiecentaur]

I am working on finding a frictionless swivel

I think a frictionless swivel may not be necessary. If the bob itself rotates with the Earth then will that matter, as long as the wire can deflect in different directions? Wouldn't any precession force be working along the direction of the wire, making it irrelevant?
But a swivel with no end stop wouldn't be necessary as the rotation is only 360° in a day. A Hook in a cup* would allow nearly 360° of movement and act as a bearing for the pendulum at the same time. and two in tandem would be even better. You can rotate the mass to the end stop at the start of the exercise.
* A jewelled point in a ceramic cup, for instance.

Another drop in the bucket of dreams for classroom demos...

Been there!

 

[Nidum]

An energiser for a foucault's pendulum can be made using a very small area electro magnet located vertical axis just below and on centre line of pendulum and a system of photo cell detectors .

As the bob comes very near but not actually to the vertical the solenoid is energised for a very short time . This in a minute way pulls the pendulum towards centre without changing it's direction of travel . This tiny impetus repeated each swing or better as necessary when swing of pendulum goes below a set value will maintain swing indefinitely and within practical limits not interfere with the natural action of the pendulum .

 

[lesaid]

I think a frictionless swivel may not be necessary. If the bob itself rotates with the Earth then will that matter, as long as the wire can deflect in different directions? Wouldn't any precession force be working along the direction of the wire, making it irrelevant?
But a swivel with no end stop wouldn't be necessary as the rotation is only 360° in a day. A Hook in a cup* would allow nearly 360° of movement and act as a bearing for the pendulum at the same time. and two in tandem would be even better. You can rotate the mass to the end stop at the start of the exercise.
* A jewelled point in a ceramic cup, for instance.

Been there!

I have found in practice, that a 'hook in a cup' doesn't work, at least, not when made out of thin brass bar. The difference in moment of inertia in the plane of the hook and perpendicular to it was enough to cause the pendulum to trace a lissajous figure, with a cycle time of 60 - 90 minutes, overwhelming the Earth's rotation. Perhaps if the hook were made of a really lightweight material? Or if the pendulum was longer and heavier in relation to the pivot piece (mine is only 1.2 metres long and 1.4 kg).

My solution was to make a 'cage' - think of the 'hook' as being a rectangular 'C' piece, with a downward pointing support from the top, and the pendulum hanging underneath. Now have three 'C' pieces, open ends joined, at 120 degree angles. That has a moment of inertia independent of swing direction (by my calculation) and was necessary to get the pendulum working. The rotation of the cage turned out not to be a problem - it didn't rotate (though it can turn very freely indeed). The swing can go any way without causing the cage to rotate at all. Presumably in an absolutely frictionless pivot, it would rotate too - but my pivot cannot be 'that' frictionless! (a ruby on a piece of glazed china).

The other crucial thing, was a very rigid support bar - even a tiny flex leads to lissajous patterns!

 

[sophiecentaur]

even a tiny flex leads to lissajous patterns!

Oh yes!
Problem with a small scale version is there are so many more significant factors. Nothing like a cathedral dome to get the job done properly. I was contemplating using the stair well of a 4 storey school building but none of my students at the time fancied it as an A level investigation.

 

[sophiecentaur]

The rotation of the cage turned out not to be a problem - it didn't rotate (though it can turn very freely indeed)

Interesting but the time constant (LR?) involved could be many days so it wouldn't matter if there were no swivel, I think.
Edit: I think that may be rubbish or irrelevant but rotation of the ball won't affect the pendulum motion; more important is the symmetry of the suspension, as you imply.

 

[lesaid]

Interesting but the time constant (LR?) involved could be many days so it wouldn't matter if there were no swivel, I think.
Edit: I think that may be rubbish or irrelevant but rotation of the ball won't affect the pendulum motion; more important is the symmetry of the suspension, as you imply.

yes - I measured the flex in the support bar, and found it had a spring constant of something like 5600 Newtons per metre (measured with a x400 USB microscope photographing the end of the bar while hanging small weights off it). Worked out that at maximum swing amplitude (3 cm), the pivot would flex sideways by something like 30-40 microns or so. After stiffening the bar, the problem disappeared.

Now its the driver I'm working on - swinging freely, it takes over six hours to decay too much to measure, so easy to observe the correct rotation. But I would like it to run indefinitely. Thinking electrostatics rather than electromagnets, but getting bogged down in the mathematics as I try to figure out equations of motion resulting from a driver that is slightly off centre. I think it might be beyond my current ability. Might need a computer simulation instead.

 

[sophiecentaur]

For a driver, perhaps consider a loud speaker style electromagnet (coaxial poles) right under the mid position. It would need a circular detector (optical?) to turn on the current briefly as the bob approaches the center. Sensing and driving would be symmetrical.
Just another way of skinning the cat.

 

[lesaid]

= This in a minute way pulls the pendulum towards centre without changing it's direction of travel ..

Is there any advantage in a brief pull towards the centre over a brief push away from the centre as the pendulum passes? Not sure why there should be - my driver pushes rather than pulls, though the driver at present upsets the behaviour of the pendulum in a way that I don't yet quite understand.

Trying to figure out how accurately aligned the driver needs to be on dead centre - and finding the maths challenging!

Wondering whether a pull-push arrangement might be more effective. A pull switching to a push as the pendulum passes overhead, might do a better job of cancelling out the effects of an offset in the driver position. But so far, that is speculation only - I'm trying to figure out if it is true or not! Experimentally, I'm getting conflicting results at the moment.

 

[sophiecentaur]

A pull switching to a push

To achieve a 'push' you would need a permanent magnet on the bob with a very symmetrical field. That is an added complication. Otoh, a coaxially built electromagnet would be fairly easy to make with steel parts (pipe and rod etc) and would Pull a steel bob.

 

[lesaid]

My 'push' is coming from electrostatic repulsion. I have a small domed electrode facing down on a 'stalk' under the bob, and a matching one on a stalk pointing up from below. As the bob passes, a spark jumps which charges the bob - it is then repelled briefly. The charge dissipates through a resistor (60 Gigohm) to ground (via a 50 gauge wire attached at the pivot point just above the ruby) so that by the time the bob swings back, it no longer repels, and is ready for the next spark. Running it at a low voltage with small electrodes ensures the spark happens as close to dead centre as possible.

The pendulum will run at a comfortable amplitude between 5 and 10 kV depending on the spark gap. I chose this approach (a) because I happened to have a controllable EHT PSU, (b) it didn't require any additional electronics or sensing, and (c) I wanted to explore a different approach just for fun. As a driver, it works fine. However, I haven't got it fine tuned enough as it does disturb the swing in an odd way that I'm trying to figure out.

I was also wary about having anything ferrous in the pendulum as there are some strong magnets around here - I didn't want to have to worry about that as a stray influence given the sensitivity of this system!

 

[sophiecentaur]

Bilmey O'Reilly - you are contemplating 10kV for a classroom demo. Have you got a written Risk Assessment and a section about the safety rail / locked room when you are not there?
I get your problem about stray magnetic fields though.

 

[lesaid]

Bilmey O'Reilly - you are contemplating 10kV for a classroom demo. Have you got a written Risk Assessment and a section about the safety rail / locked room when you are not there?
I get your problem about stray magnetic fields though.

actually - I think I suggested using the pendulum free-swinging without a driver for the demo in my first post on this! A six hour useable swing time gives plenty of opportunities to measure the rotation. Though the supply only needs to provide a few tens of nA with a very small capacitance. Do secondary school physics classes not have access to safely current limited supplies of a few kV?

thankfully though - I'm not a teacher - I'm a hobbyist so I don't have to worry about written risk assessments! (been there, done that, in other contexts!).

I was wondering - your A-level students don't find Foucault pendula interesting? I'm curious - what kind of projects do they favour? electronics and robots?

 

[sophiecentaur]

I was wondering - your A-level students don't find Foucault pendula interesting?

That's ancient history now. They seemed most interested in the so called Cosmology bit of the course, mainly and there was not a lot of practical work for that - except for estimating distances out on the school field. Distance from the science block in Parsecs etc.. (Suitably scaled)
On the safety front, the High voltage supply was perfectly 'safe' but I had one lad who got a small belt off the supply and he made a real fuss about it.

 

[lesaid]

On the safety front, the High voltage supply was perfectly 'safe' but I had one lad who got a small belt off the supply and he made a real fuss about it.

Actually - from what I know now about the tiny impulses necessary, I should think that a reasonably strong electromagnet and a few iron filings at the bottom of the bob might be enough - and not enough to be affected much by more distant magnets! If fraction of a second of repulsion between two 4 mm diameter domed bolts at 5 kV is enough to get a good swing - it won't take many iron filings with a decent magnet!

But this way seems simpler, if I can get it to work symmetrically!

 

[sophiecentaur]

But this way seems simpler, if I can get it to work symmetrically!

I would think that an electromagnet with a carefully wound coaxial arrangement could be arranged to be vertically below the top fixing. Your 'few iron filings' could do the trick. You could investigate the actual effect of your weird fields by comparing two bobs - one ferrous and one non-ferrous. (perhaps you've done this already; you seem to have done quite a lot of prep work on this.)
The project sounds serious fun.