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Do spinning objects really lose weight?

  1. Jan 28, 2009 #1
    Several years ago there were widely reported claims that gyroscopes and the like became lighter.Although these claims have generally become discredited could there be any truth to them and could proof come from theoretical reasoning?
     
  2. jcsd
  3. Jan 28, 2009 #2
    Might be a myth you know.
     
  4. Jan 29, 2009 #3

    turin

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    Of course. However, science does not pose speculation as theory. I know of no theory (in the scientific sense) that suggests that gyroscopes become lighter when they spin. However, I am not confident enough in my knowledge of GR to be certain of the lack of theoretical support. There are some wierd things hiding in the seemingly simple theory of GR.
     
  5. Jan 29, 2009 #4
    Perhaps any proof could come more readily from Newtonian gravity which must conform to GR within its own domain and perhaps the proof could be very simple.What if I posed a seemingly speculative statement which is that according to Newtonian gravity a spinning gyroscope should display a weight loss.Whether this is measurable or not depends on several factor which include the sensitivity of the weighing instrument and the structure and spin speed of the gyroscope.
     
    Last edited: Jan 29, 2009
  6. Jan 29, 2009 #5
    Only if it has wings such that it acts, slightly, like a helocopter.
     
  7. Jan 29, 2009 #6

    jambaugh

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    As a matter of SR and GR, a spinning object having more energy=mass than its stationary counterpart would weigh slightly more.
     
  8. Jan 29, 2009 #7

    A.T.

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    Rotating objects have a lot of momentum stored, that can be used to lift the object, creating the illusion that the object is very light.
     
  9. Jan 29, 2009 #8

    Andy Resnick

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    Is the OP's question in regards to Podletkov's "results"?
     
  10. Jan 29, 2009 #9

    Vanadium 50

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    I think this is the "Tohoku Top" experiment, Hayasaka et al. Phys. Rev. Lett. 63, 2701 - 2704 (1989). It was repeated with a null result by Nitschke et al. Phys. Rev. Lett. 64 2115 (1990), and Faller et al. Phys. Rev. Lett. 64 824 (1990).
     
  11. Jan 29, 2009 #10
    Even disregarding relativistic and aerodynamical effects Newtonian mechanics may predict that a weighing instrument can,in principle, detect a weight loss and the proof may be quite simple.In terms of SR when the gyroscope gains mass/energy something else loses it . There have been experimental attempts to measure weight loss but none have been conclusive.
     
    Last edited: Jan 29, 2009
  12. Jan 29, 2009 #11

    turin

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    The amount of speculation is irrelevant. This is pointless unless you can show it, either by simulation, or analytical proof, or something. That's one of the beauties of science: it uses math, so predictions of a given theory can be made very precise. When you use math to make a claim based on a theory, we don't call it a speculation, we call it a testable prediction. Please show us the mathematical evidence that Newtonian gravity predicts weight loss of a spinning gyro. I make this request sincerely, because, if it is true, I find it very interesting, but I don't see how to extract this prediction from Newtonian gravity.
     
  13. Jan 29, 2009 #12

    Vanadium 50

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    I just posted two references showing it isn't there and that the Tohoku experiment was wrong. Did you read them?
     
  14. Jan 29, 2009 #13
    In a spinning object angular momentum is acting 90 degrees from the base. if the objects mass is spinning and even a part accounted for through spinning then perhaps the object would weigh less although its mass remains the same.
    This seems an obvious result and the fact that it is not part of established physics with its own formula suggests to me that it mus be very difficult to model both mathematically and empirically or that I have looked at it too simplistically. Like Turin I would like to see the maths.
     
  15. Jan 29, 2009 #14
    Sorry I dont know how to present diagrams or mathematical equations.I will illustrate this with the simplest example I can think of-a point mass being whirled in a horizontal circle by a light inextensible string.Normally, when a free body force diagram is drawn and if air resistance is ignored ,there will be two forces acting on the mass
    1.the tension in the string which acts along the line of the string where it joins the mass.
    2.The weight of the mass which acts vertically downwards
    If we carry out the conventional analysis we would describe that the vertical component of the tension supports the weight of the mass and that the horizontal component of the tension provides the centripetal force and no surprises are revealed.
    Now we need to be a little bit more fussy and look at our free body force diagram again.The weight,in fact,does not act vertically downwards it points towards the earths centre.Because of this the weight provides some of the centripetal force,the tension will be smaller and there will be an apparent but not a real weight loss.Obvious but if there is a mistake in my reasoning or if this has been done before I apologise.A couple of extra points
    1.The above analysis can be extended for a gyroscope as it is usually imagined.
    2.The biggest fractional weight loss(everything else being equal) should be recorded by the largest diameter gyroscope with most of its mass being concentrated in a thin ring on its circumference.
    3.The spinning object somewhat resembles a satellite and if its plane of orbit coincides with a central plane of the earth and if its spin speed has the right value for its height above the earth then the apparent weight loss will be total.You could cut the string and watch it go.I wouldnt try this because you will need a very long piece of string and a very long ladder.
    (If I havent made my presentation clear sketch a free body force diagram with a round earth)
     
    Last edited: Jan 29, 2009
  16. Jan 29, 2009 #15
    The weight is a vector, pointing vertically down.
     
  17. Jan 29, 2009 #16
    Weight is a force,it is a vector and its direction is towards the earths centre of gravity.
     
    Last edited: Jan 29, 2009
  18. Jan 29, 2009 #17
    How does that differ from vertically down?
     
  19. Jan 29, 2009 #18
    Hello Phrak,it was your previous comment that made me realise that I could have made my presentation clearer and so I edited it by adding an extra sentence in brackets.Have a look try it out and that should answer your question.
     
  20. Jan 29, 2009 #19
    lol. the point mass is partially orbiting the center of the earth. I never thought of that.
     
  21. Jan 29, 2009 #20

    turin

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    The difference being? Is this untrue for an object that is not spinning?

    Why is there an apparent weight loss? I see no apparent weight loss here.

    I guess by "extended" you mean "changed completely". I see no connection between lessening of tension in the string and lessening of weight of a spinning gyro.
     
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