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Fake Cavendish experiments

  1. Jul 22, 2009 #1
    I've seen redone Cavendish experiments on YouTube where the 2 lead spheres suspended from a latter rotate to to slightly larger lead stationary bricks. The spheres rotate and touch the bricks. This must be fake. Gravity is just not that strong a force. My guess is that equilibrium is attained by a deflection on both parts in the hudreds of thousanths of an inch or less.
    Would my critique be correct?
    Squarkman
     
  2. jcsd
  3. Jul 22, 2009 #2

    negitron

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    I'm inclined to agree with you. In the original experiment, Cavendish measured torsional forces, not displacement--I don't believe there was sufficient displacement to be measure with any accuracy, certainly not that that time. Never mind enough you could actually watch.
     
  4. Jul 22, 2009 #3
    Here's the actual video and all comments. It's so fake.
     
  5. Jul 23, 2009 #4

    Vanadium 50

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    I would think that before you call someone a liar, you'd have a stronger case than "It's so fake".

    You didn't mention what video, so let's consider http://www.youtube.com/watch?v=QquPcufXDUo":

    https://www.youtube.com/watch?v=QquPcufXDUo

    Now, how long do we expect it takes the test weights to "fall" towards the large ones? For spheres near their surfaces, [itex] g \sim \rho R[/itex] and [itex]t \sim 1/\sqrt{g}[/itex] so [itex]t \sim 1/\sqrt{\rho R}[/itex]. A 3 pound ball of lead has density twice that of the earth and a radius 200 million times smaller. So, you'd expect it to take 10,000 times longer for it to fall.

    On earth, it takes an object about 1/7 of a second to fall 4 inches. So on the video you would expect it to take about half an hour. It takes 40 minutes.
     
    Last edited by a moderator: Apr 24, 2017
  6. Jul 23, 2009 #5
    I guess you've made a believer out of me but I'm still have a few concerns. What about all of the much more massive object just a bit farther away that the lead sinkers that would tend to pull the rotating masses in the opposite direction. This demonstration was not done in outer space away from other planets, masses, etc.
     
  7. Jul 23, 2009 #6
    I'm going to try the same experiement on ice and see if a huge lead ball will drag a smaller lead ball toward it on a slick chunk of ice.
     
  8. Jul 23, 2009 #7

    mgb_phys

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    You can't really have a torsional force in a wire without a displacement of the thing on the end of the wire.

    Cavendish hung a mirror on the wire, by looking at a scale reflected in the mirror he was able to see a very small rotation of the wire caused by the real physical motion of the masses on the end of it.

    For the video - you would need whatever the beam is balanced on to be seriously low friction and a bubble level like that is nowhere near accurate enough to say that it is flat. To convince anyone you should move the lead blocks to the other side and have them move the bar back in the same time.
     
    Last edited: Jul 23, 2009
  9. Jul 23, 2009 #8
    The torsion would have been much greater had he set it up with two spheres which rotate by the torsion as in your demonstration. Why didn't he do that. Then you'd get a greater accuracy, right?
     
  10. Jul 23, 2009 #9

    negitron

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    Except the Cavendish balls aren't in freefall, they have to act against the torsion of the suspending line, which increases linearly with angular displacement. Given that the gravitational force between the masses amounts to on the order of 10-7 N, I can't see how it can act through an arc of the size shown in the video. Consider that in Cavendish's original experiment, the linear displacement of the end of the rod the moveable masses were suspended from was only .16" according to the Wiki link on his experiment. And that was performed ounder much, much more rigorous conditions than shown in the YouTube vid.

    Sorry, but YouTube videos are not ever to be taken at face value. Ever.
     
  11. Jul 23, 2009 #10

    negitron

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    Note that I said "sufficient displacement."
     
  12. Jul 23, 2009 #11
    So negitron, you are debunking the YouTube video?
     
  13. Jul 23, 2009 #12

    negitron

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  14. Jul 23, 2009 #13
    As am I! Thank you!
     
  15. Jul 23, 2009 #14
    I am unable to see a link to the video mentioned by the OP, but I have no reason to believe that a homemade demonstration would not be possible. rockyshephear and negitron, do you also believe that this one is fake?
    http://www.fourmilab.ch/gravitation/foobar/
     
    Last edited: Jul 23, 2009
  16. Jul 23, 2009 #15
    I'll review it and get back to you. Any of you other physicists want to chime in on this one? Should be terribly easy question for you!
     
  17. Jul 23, 2009 #16
    I would like to say that I don't disagree that everything in the universe attracts everything else. I am just skeptical that demonstrations like we are discussing can create near frictionless environments to demonstrate their point. I am positive if a lead sphere 100 pounds we 10 feet from a 1 oz lead ball in a universe where there were no other masses, the object would eventually come in contact with the larger sphere. No doubt whatsoever. I just don't see how a home-made setup can reduce the effects of friction, create such delicate balance required, etc to successfully assimlate the 'out in space' setup I just described.
     
  18. Jul 23, 2009 #17

    mgb_phys

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    The example in TurtleMeister's post is certainly possible. We did an undergrad demonstration that is very similair, although without the water brake because to actualy measure G you need the stiffness of the wire and so it has to oscillate freely without the large masses present.
    The first video looked like the beam was sitting on a pad of something - but if that was just a water brake as in the fourmilab one then it looks right.
     
  19. Jul 23, 2009 #18
    Good point; typically you'd have to take into account the torsion coefficient of the wire which gives it a 'natural oscillation'....and since you do not know that value in the 'hanging thread' experiments, one cannot make an accurate prediction.....meaning you cannot make an acceleration prediction in favor of OR AGAINST the video without knowing the torsion coefficient of the hanging wire (thread).

    As 'mgb phys' pointed out above, measurements need to be taken in free oscillation first (without mass attractors) in order to obtain the natural oscillation. That is what Cavendish did, and then, if I'm not mistaken, he took the measurements with mass attraction while it was still oscillating, and then compared the difference in the oscillation displacements, which is where the .16'' came from.

    This video here is probably better because it shows clearly the oscillation (in time lapse video).

    http://www.fourmilab.ch/gravitation/foobar/figures/movie-1.mov [Broken]

    Creator
     
    Last edited by a moderator: May 4, 2017
  20. Nov 17, 2009 #19
    Hey folks, this is my first visit and post on these forums. I hope no one minds that I dig up this old thread. I found it while doing some searches on fake Cavendish experiment videos. I've seen several videos on Youtube including the ones here that show dramatic visual displacements in a short amount of time. I was skeptical, as most seem to be here. I decided to do some experimenting of my own, and I whipped up a crude torsion balance using fishing line, a wooden dowel, large lead sinkers (just guessing they are about 1/4 kg) on the end of the dowel and weights (15 pounders) from my weight bench. I hung the balance from my ceiling in the hallway so the balance was just above the floor.

    It took about an hour for the system to settle, apparently the fishing line had some twists in it. I did not use any dampener (water brake). When I positioned the large masses near the balance, I got about 1 cm of displacement in about 1 minute. I was shocked! I thought that surely this was due to some static charges. However, my experience with pith balls in electrostatics tells me that when contact was made, there should be some repulsion, but that didn't happen, the small and large masses remained in contact. I could then easily reverse the system and again got about 1 cm of displacement in about 1 minute time (should happen twice as fast right, but I wasn't timing anything). Maybe the use of a really long fishing line, from my ceiling to my floor, means that there is very little torque needed in twisting the fishing line? What do you folks think about that?

    Oh, by the way, I'm a high school physics/mathematics teacher.
     
  21. Nov 17, 2009 #20
    Welcome to PF Broz. I never doubted the fourmilab video. I'm not saying I believe everything I see on Youtube. But this one seemed completely plausible to me. Very good that you did your own experiment. I've thought about doing it myself. But the degree of sensitivity I would like to achieve would be very difficult for an amateur with limited resources. If you're interested in gravity and torsion balances then you may find these links interesting.
    http://www.npl.washington.edu/eotwash/intro/intro.html [Broken]
    http://www.physics.uci.edu/gravity/
     
    Last edited by a moderator: May 4, 2017
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