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I Foucault's Pendulum Recreation for Physics Project

  1. Oct 26, 2016 #1
    (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!
    Best,
    Brad
     
    Last edited: Oct 26, 2016
  2. jcsd
  3. Oct 26, 2016 #2

    Vanadium 50

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    And that is the key question in your project.
     
  4. Oct 26, 2016 #3

    Andy Resnick

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    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
     
  5. Oct 26, 2016 #4
    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!
     
  6. Oct 26, 2016 #5

    Nidum

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

    You can buy many varieties cheaply in tool stores and online .
     
  7. Oct 27, 2016 #6

    Andy Resnick

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    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.
     
  8. Oct 27, 2016 #7

    jbriggs444

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    https://missionbelt.com/products/unobtainium (tongue deeply embedded in cheek)
     
  9. Oct 28, 2016 #8
    what would you suggest I use as alternative materials?
     
  10. Oct 28, 2016 #9
    Last edited: Oct 29, 2016
  11. Oct 29, 2016 #10

    Tom.G

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    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.
     
  12. Oct 29, 2016 #11
    Does anyone have a recommendation for another bob? or what about a swivel that works pretty well it does not have to be perfect.
     
  13. Oct 29, 2016 #12

    Tom.G

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  14. Oct 30, 2016 #13
    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:

    upload_2016-10-29_22-59-10.png
     
  15. Jul 4, 2017 #14
    Hi Brad, interested to hear how you went? Especially in how you implemented the 'kicker'.
     
  16. Jul 12, 2017 #15
    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.
     
  17. Jul 12, 2017 #16

    Vanadium 50

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    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.
     
  18. Jul 12, 2017 #17
    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.
     
  19. Aug 1, 2017 #18
  20. Aug 1, 2017 #19
    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!
     
  21. Sep 18, 2017 #20
    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.
     
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