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Balance proves gravity isn't a force

  1. Aug 11, 2010 #1
    consider a weight balance with two unequal masses on the two arms.It comes to rest after tilting a few degrees. If one of the masses really experienced a greater force, then there is no reason why the balance shouldn't tilt 90 degrees
     
  2. jcsd
  3. Aug 11, 2010 #2

    bapowell

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    What kind of scale is this that comes to rest after tilting a few degrees? I'm not a scale expert, but I think the kind of scale you are envisioning has a counter-weight on it, that you adjust to achieve equilibrium.
     
  4. Aug 11, 2010 #3

    HallsofIvy

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    Only because there is friction in the pivot. If you have a a balance with no (or very little) friction it will tile 90 degrees.
     
  5. Aug 11, 2010 #4

    Ich

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    They place the center pivot a little bit higher than the eyes where you attach the pan. (http://visual.merriam-webster.com/images/science/measuring-devices/measure-weight/beam-balance.jpg" [Broken]).
    That proves that they know how to build balances, but nothing about gravity.
     
    Last edited by a moderator: May 4, 2017
  6. Aug 11, 2010 #5
    I think other posters have already answered it, but I'm curious:
    If gravity isn't a force, what is gravity?
     
  7. Aug 11, 2010 #6
    Gravity is the effect of curvature of space-time rather than a force.
     
  8. Aug 11, 2010 #7
    To add:
    In non-intertial reference frames, gravity can be treated as a resulting psuedo-force, on the same level as the centrifugal and coriolis forces.
     
  9. Aug 11, 2010 #8

    ZapperZ

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    We shouldn't derail this into something it isn't. The OP clearly isn't aiming along this direction, or at least the "evidence" being used clearly doesn't deal with "gravity not a force and merely a curvature of space-time".

    This is an elementary problem based on rather incompletely understanding of a situation. Let's not take it beyond that. There are already many threads dealing with gravity within the GR description. Continue that line of discussion in those appropriate threads.

    Zz.
     
  10. Aug 11, 2010 #9
    Is it the friction of the pivot? If that was the case then the friction would have to be a function of the angle right? Otherwise, could it be that the lever arm length changes? hmm...
     
  11. Aug 11, 2010 #10
    @apolski and espen180: That question was aimed at the OP.
    He evidently has some basic conceptual mistakes - let us not confuse him with general relativity, since it will not solve his problem : the body with the largest mass will feel the largest force (or pseudoforce, or whatever).


    @OP: Study torque and angular momentum. Than you will see that the more the balance distances from the horizontal, the least will be the tendency to go up to 90 degrees. If the balance has some friction in it - which is likely to be the case - friction will equilibrate that tendency and the balance will be in equilibrium before the 90 degrees. Sometimes, the most stable position isn't achieved because some constraining (normal) or friction forces are in it. The balance that doesn't tilt completely is analogous to a book that doesn't fall because he is on a table.

    Have I convinced you, vin 300? Let me give a perhaps simpler reason. You argued that if both masses had different weights, the rod should tilt to 90 degrees, and since that wasn't the case, both masses should have the same weight. Since you can put any 2 masses in that scale, this would mean that everything weights the same. So, by your reasoning, one apple weights the same as 100 apples - clearly, something is wrong.
     
  12. Aug 11, 2010 #11

    russ_watters

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    It's not friction, it's the lever arm changing due to the geometry of the scale. Ich had it right.
     
  13. Aug 11, 2010 #12
    Try that experiment in a vacuum. Good luck with that.
     
  14. Aug 11, 2010 #13
    Yep. If the pivot is directly inline with the two different masses, the greater mass will cause a full and complete 90 degree tilt.
    The pivot in this case is not directly inline, rather above the two masses; thus becomes a "balance"
    As russ said, geometry plays the critical role.
     
  15. Aug 11, 2010 #14
    A vacuum would have no substantive effect on outcome.
     
  16. Aug 11, 2010 #15
    I don't know what kind of scale we are talking about. Imagine a beam that is pinned at its center and freely rotates. Imagine two pans are attached at the extrema of the beam so as to rotate freely like cars on a ferris wheel. Then the beam would go vertical. Indeed it would be very difficult to manufacture such a scale so that it didn't go vertical even with no weights on it.

    Although friction might help to retard this effect, I doubt it would help much unless the two weights were very similar. I expect that good balance scales reduce the friction as far as is practical anyway. Instead, I expect that they have some mechanical means of preventing the beam from rotating freely beyond a certain angle, while allowing it to rotate freely when less than that angle. In that case, a difference in weight would cause the scale to rapidly reach that limiting angle. Does someone here have access to a balance scale and can confirm these conjectures?

    Here is an example of a balance scale.
    http://www.findingking.com/popup.aspx?src=images/Product/large/s-757_freeshipping.jpg"
     
    Last edited by a moderator: Apr 25, 2017
  17. Aug 11, 2010 #16

    Clue: The weights are actually BELOW the pivot center.
     
    Last edited by a moderator: Apr 25, 2017
  18. Aug 11, 2010 #17
    Yes. I'm talking about the vacuum of space. Gravity is the only force acting on the objects in space. They are in a free-fall. That was my whole point in stating: "good luck" - as in good luck trying to prove gravity isn't a force.
     
  19. Aug 11, 2010 #18
    Ah, OK :smile:
     
  20. Aug 11, 2010 #19
    :tongue:
     
  21. Aug 11, 2010 #20

    russ_watters

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    Sounds like people are still having trouble visualizing this. See this image: http://www.cdc.gov/diabetes/pubs/images/balance.gif

    Picture a slight rotation clockwise. When rotating slightly clockwise, the pan on the right gets closer to the fulcrum horizontally while the pan on the left gets further away, thus eventually finding a torque equilibrium.

    Note, this is also why using a flexible pole vastly increases stability for a tightrope walker.
     
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