Can anyone suggest a suitable material for a Foucault pendulum platform?

[Tom.G]

As @NTL2009 pointed out, a Mercury bearing sounds ideal. Even old (circa 1910) Astronomical Observatories used them.

The new mounting incorporated many of the features of the successful 60-inch telescope, including the mercury flotation bearings to support the 100 tons of moving telescope that had to track the stars so precisely and smoothly.
​

From: https://www.mtwilson.edu/building-the-100-inch-telescope/

A Google search: https://www.google.com/search?&q=mercury+bearing turns up many hits, most but not all are Mercury brand

 

[NTL2009]

Would a blade shape (I get eerie pictures in my mind of "The Pit and the Pendulum"!), rather than a more spherical bob weight help to reduce air drag? In turn, that might allow you to reduce the weight of the pendulum and still get the same amount of swing time? Less weight would help to reduce the pitting issues you see.

Obviously, that has the risk of being biased in one direction, but if you reverse the direction each experiment, that should null it out?

 

[lesaid]

Would a blade shape (I get eerie pictures in my mind of "The Pit and the Pendulum"!), rather than a more spherical bob weight help to reduce air drag? In turn, that might allow you to reduce the weight of the pendulum and still get the same amount of swing time? Less weight would help to reduce the pitting issues you see.

I see a problem with that - this pendulum has to be able to swing in any orientation, and the pendulum shaft and pivot assembly do not themselves rotate with the swing. So, there would need to be a mechanism to keep the blade pointing in the direction of swing. Not sure how to do that without a lot of complexity and probably electronics!

BUT you have reminded me that I should try to estimate the energy loss due to air resistance. It won't be much because the motion of the bob is so slow - it only has a small swing arc of at most about 3cm amplitude. Then I'll have some idea how much effort it is worth going to to reduce it.

Actually - that might be interesting - I can think only of three significant mechanisms for energy loss - air drag, friction on the pivot and twisting/bending of the pivot support assembly. I can easily work out the energy lost overall from the decay curve, and I can probably put an upper bound on the energy lost by distorting the support assembly, so this might indirectly give me an idea how good the pivot is!

 

[lesaid]

As @NTL2009 pointed out, a Mercury bearing sounds ideal.

I like the idea, but I don't see how it would be practicable for me. In the UK, mercury is not available to the general public - I would have to get a government license to be allowed to buy it as a hazardous chemical. I'm also a bit nervous about it because this pendulum is operating for long periods in a confined space with little ventilation (to minimise air currents that might also affect things). If there is significant mercury vapour, it is likely to build up. I spent some time looking searching on the internet and found some interesting links, but nothing offering an 'off the shelf' bearing that I can buy and use simply without worrying about the mercury within.

I'm interested in the idea though - and wonder how the friction would compare to the rolling pivot that I have now in terms of energy loss. Perhaps there is a trade-off to be considered between efficiency (overall swing time) and symmetry.

I suppose - if I wanted to experiment with this kind of bearing, that gallium, with a small heater, would be the next best thing, if perhaps rather expensive. Might be fun to try!

Before changing direction so completely though, I think I should exhaust the possibilities in what I have already! I know the pivot works well - sometimes !

I am also revising my thoughts about the pivot tip. It occurs to me that a smaller radius tip should be less affected by any slope on the pivot. So I'm thinking again about the carbide 'lathe tip' and diamond stylus ideas. I don't know how to go about calculating their strength against the forces involved though - might be a case of trial and error to find out what breaks and what works! Doing some more calculations to quantify the relationship between slope and forces on the system.

Thank you for the suggestions!

 

[NTL2009]

Ahhh, a 3 CM swing would seem to make air resistance a tiny, tiny effect. So I doubt that a 'blade' shape versus spherical would matter. But then again, these forces are so small, I really don't know.

As far as orientation, just curious, but doesn't your pivot rotate with the pendulum orientation now? I'm not sure a blade would change anything in that regard (though it still is probably not worth pursuing).

 

[lesaid]

As far as orientation, just curious, but doesn't your pivot rotate with the pendulum orientation now? I'm not sure a blade would change anything in that regard (though it still is probably not worth pursuing).

I'm not quite sure why it doesn't rotate - I thought that it should and was doubtful if my current pivot design would work at all for that reason. If it rotated, I would be very limited in the swing time I could get before the pivot frame collided with the support. I'm guessing it is because there is very very little friction on the pivot since it rolls rather than slides, and a reasonably high moment of inertia around the pendulum, shaft axis.

The cage is very free to rotate - so much so that when I experimented with an electrostatic drive, so that the pivot cage was at a few kV potential with respect to the support arm, after a few minutes swinging, the cage rotated until it collided with the arm and short-circuited the (very high impedance) supply! The electrostatic attraction between a few kV and ground is miniscule, but enough! That problem was sorted with a change to ensure the pivot and the support were both always at ground potential and the charge only was applied to the bob. That in fact was the change that led to my current symmetry problem. I have changed it back again, but the symmetry problem is still here unfortunately!

There is more on that in the earlier thread (referenced in the first post of this thread) if you are interested. Or ask and I'll explain here.

 

[lesaid]

I have some carbide lathe tool tips , triangles about 1cm on a side but they've a hole dead center for mounting.

I took a look at the link you posted - I'm wondering if those tips actually offer a symmetrical point rather than an 'edge'?

However, I came across this https://www.screwfix.com/p/vitrex-tile-scribe/70830?tc=FB9&ds_kid=92700022861140982&ds_rl=1249796&ds_rl=1245250&ds_rl=1249481&gclid=EAIaIQobChMI1-LA0dq_2wIVY7XtCh1enARxEAQYBCABEgLZQvD_BwE&gclsrc=aw.ds&dclid=CP_chdvav9sCFYcB0wodWpsLcw

So, at that cheap price, I'll pick one up, take a close look at the tip and see what shape it is. I'm guessing it might do the job nicely, but I can't see it working with my glazed plate platform. I wonder how it would do on the 'gorilla glass' mentioned earlier.

But before trying anything else, I want to find and resolve the symmetry problem I have just now, then assess where I am, and change one thing at a time !

Thank you

 

[Tom.G]

If you want to avoid a heater, here are some other room temperature liquid metals to look into:

https://en.wikipedia.org/wiki/Galinstan#Physical_properties
http://www.indium.com/thermal-interface-materials/other/liquid-metal/

Above found with: https://www.google.com/search?&q=room+temperature+liquid+metal+alloy

 

[lesaid]

If you want to avoid a heater, here are some other room temperature liquid metals to look into:

https://en.wikipedia.org/wiki/Galinstan#Physical_properties
http://www.indium.com/thermal-interface-materials/other/liquid-metal/

Above found with: https://www.google.com/search?&q=room+temperature+liquid+metal+alloy

Just did some calculations. If I'm correct, and assuming a spherical bearing fitting inside a spherical 'container' with a galinstan fluid, the inner hemi-spherical piece would need to have a radius around 6 cm (to support up to 2 kg (the bob plus the shaft and pivot cage assembly). A diameter of 12 cm would need fabricating a new cage and is a good deal larger than I was hoping for. Mercury would reduce this to a bit under 5 cm radius, which is still large.

So I think this will have to stay for now as a 'plan B' if rolling ball pivots cannot be made to work. Thank you all for the intriguing idea!

I'm continuing work on a mathematical model of this to establish how much difference the radius of the ball and any slope of the platform might make. I am hoping that I can dispense with precise levelling and only have to get the support sufficiently rigid and approximately flat and level.

 

[Scottsgirl98]

Hello! I was wondering if anyone could assist me with a project I have for my upper division physics class. The class requires me to build a fully functioning Foucault pendulum that is reasonable in size. I would like to build one with a wooden base and sides that has a square base of about 1 ft by 1 ft and 3 ft tall (these are flexible within 0.5 feet in each direction), aside from this I have no clue how to start because there seems to be a lack of information about how to build a relatively small Foucault pendulum on the internet.

My main questions are what type of string do I need, how do I need to hang it, and what type of pendulum bob do I need? Also, if it works properly, will it really just start moving by itself? Thank you!

 

[Charles Link]

See https://www.physicsforums.com/threads/foucaults-pendulum-recreation-for-physics-project.890835/

 

[Scottsgirl98]

I'm about to pick up again on a project started last summer to build a Foucault pendulum that will fit on, or at least, above a desk in an ordinary room. This was described, with photographs in https://www.physicsforums.com/threa...or-physics-project.890835/page-3#post-5845824

The pendulum (about 1.2 m in length with a 1.5 kg bob) pivots on a 1mm ruby sphere at the tip of a 'probe', that rests on a hard platform. The hardest surface I have found so far is a piece of glazed dinner plate - but it seems this is not hard enough. After some time running, the platform is pockmarked with tiny pits, which I suspect will be influencing the swing.

Does anyone have any ideas for a material that is

• very hard (harder/stronger then glazed china)
• very smooth
• readily available without being too expensive
• can be cut or broken into a piece that provides a roughly square/circular platform around 3 cm across with a very flat top surface and not more than about 1 cm thick
• Will support a weight in excess of 1.5 kg on the 1 mm ruby sphere

I have read of 'ceramic glass' but not so far found a source that was both affordable and from which I could create the necessary size and shape of platform.

I had the pendulum working well last summer, running for nearly six hours before the (unpowered) swing decayed too much to establish its direction, and showing rotation at the expected rate for my latitude. I then disassembled it to make some modifications but on reassembly, it no longer worked (swing settled into a preferred direction over three or four hours). I think the problem lies in the structure that supports the pivot, so my first task will be to design a more robust (very rigid but finely adjustable) version.

Other thoughts or ideas welcome!

Can you give me some dimensions and the types of materials used for your pendulum? I am trying to make one for a class project and have no clue where to start unfortunately. Thank you so much!

 

[Paul Colby]

It's late, I haven't read the entire thread which looks interesting. Just one question. Is a mechanical direct contact bearing the best (lowest loss) option? Two things come to mind, a flexure or an air bearing. Fabrication cost, complexity are all real issues so this is more of a theoretical question.

 

[Vanadium 50]

1. The intent of the assignment is surely not "ask some people on the internet to do the research for you". This makes me crabby.
2. Spamming the forum with multiple requests for us to do #1 also makes me crabby.
3. Some of your questions can be answered by building a regular pendulum. I would recommend that as a starting point.

 

[anorlunda]

[Moderator note: A second thread on this same topic was started yesterday. Posts in that thread were merged into this one.]

 

[Charles Link]

1. The intent of the assignment is surely not "ask some people on the internet to do the research for you". This makes me crabby.
2. Spamming the forum with multiple requests for us to do #1 also makes me crabby.
3. Some of your questions can be answered by building a regular pendulum. I would recommend that as a starting point.

I think it could be very useful for 3 or 4 people to try to make this a group effort. The project is difficult enough, that there are many who have said that it can't be done.
And for a good book on this topic, see https://www.amazon.com/dp/0743464796/?tag=pfamazon01- years 20 I read this book about 10 years ago. It is very good reading, and it even has the derivation of Foucault's equation for the period as a function of latitude in the appendix. If I remember correctly, it is $T=\frac{T_o}{\sin{\theta}}$, where $T_o=24$ hours. I believe in the book, it said Foucault simply had an M.S. degree, and other famous mathematicians and physicists of that time did not have an answer for the period of his pendulum.

 

[Charles Link]

[Moderator note: A second thread on this same topic was started yesterday. Posts in that thread were merged into this one.]

The first one on the do-it-yourself Foucault pendulum began May 15, 2018. It seems to be a very good idea to merge the two threads. It looks like the previous participants are very happy to share their experiences.

 

[JBA]

Depending on the size of the surface you need you might consider one of the below sapphire discs. in smaller diameters they are very inexpensive .ie. the example : .250 in diameter shown is $7.60 for 1 pc. Both ruby and sapphire have a hardness of 90 on the mohs scale.

https://www.swissjewel.com/product/sapphire-windows/w6-30/

 

[lesaid]

Just discovered the recent posts here! I've been away from this project for some months (studying other things) and will be returning to continue my efforts some time in June or July. In the meantime, I am happy to exchange ideas and experiences with others engaged in the same kind of thing.

One of the recent posts suggested building a regular pendulum as a starting point. I started by using household string tied to a hook in the ceiling to suspend a bag full of nuts, bolts and other metallic scraps, including a small reel of solder if I remember right. Anything I could find to make a decent weight! Of course it was useless as a Foucault pendulum but it was a good starting point, and taught me a lot about how free pendulums swing (and how string 'unwinds' under tension!). Then proceeded to improve the pendulum, one component at a time until I arrived at the one described earlier in this thread.

Next step will be to rebuild the suspension platform to be much more rigid and finely adjustable for level, and explore using an off-the-shelf ruby bearing as the pivot support platform (if it is available at a sensible cost). Along with trying to develop a better mathematical model of how the pivot should behave on a sloping platform, and on a ruby bearing with a concave supporting surface. I'm a bit nervous as to how much force I can apply to a hard ruby ball 1 mm in diameter resting on an equally hard ruby surface without the ball cracking, but not sure how to calculate it so I'll have to try it and see!