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Why don't we feel the gravity of other objects?

  1. Jul 28, 2013 #1
    Since every mass attracts every other mass according to Newton's universal law of gravity, why don't I just get pulled towards my computer monitor?

    Now you might say, the gravitational constant is so small that the force between you and the computer is amazingly low for any noticeable effects. All that tiny amount of pull will do is create a tiny amount of friction between you and the chair you're sitting on so as to make the net force zero. But here's a thought experiment, suppose I put two blocks near each other on frictionless ice. And suppose there is no other object around so that the majority of the force they will feel horizontally will be due to the other block. Will the two blocks slowly move towards one another?

    For example, suppose I take two incredibly dense blocks that have masses of 1000 kg, and I place them 1 m apart.
    Then the force that each will feel due to the other block is F= 6.67*10^-11 * 1000* 1000/ (1)^2=
    6.67*10^-5 N, which gives an acceleration of 6.67*10^-8 N for either block. Seriously low, but in under 1 hour and a half these two objects should have made it towards one another. See for yourself, delta x= at^2/2, 1= 6.67*10^-8 * t^2/2, t^2= 2/(6.67*10^-8), t= 5476 seconds. Now this number should even be less because acceleration would also be getting bigger since the two objects would be getting closer and closer.

    Is this really the case, if there really was 0 friction between the blocks and the ice, like on some sort of frictionless air hockey table, the two blocks would move towards one another? Or what are we overlooking?
     
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  3. Jul 28, 2013 #2

    ZapperZ

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    There are some experiments, such as the very sensitive torsional experiments that tries to measure the value of the gravitational constant G right down to the sub-micron scale, in which just a movement of a person in the lab near the apparatus can affect the measurement.

    The question is whether there's something sensitive enough, and in enough isolation, to be able to detect such minute effects. The example you gave requires not only that friction is practically zero, but no other forces around that can disrupt the force of attraction between the two blocks. In includes stray fields either externally, or from the block itself. Now, you'd think that this is trivial, but it really is not when you are trying to detect very, very small effects!. Even air movement in an apparent still room can easily disrupt any such effects! You also have to make sure your block not only stays electrically neutral, but it stays electrically neutral through the whole thing (object have been known to pick up stray charges, especially from cosmic background radiation).

    So those are what YOU have overlooked.

    Zz.
     
  4. Jul 28, 2013 #3
    Thank you, I was thinking of doing an experiment in a room where the air has been sucked out. Suppose they're made of unreactive materials, so the electrical effects are negligible too. Then if we could get coefficient of friction to be precisely 0, the blocks would move towards one another right? But if there is even a small amount of friction then since the blocks are heavy (1000kg), the frictional force is uN, which would give it more then enough force to cancel the gravity of the other object right?
    Interesting about the torsional experiments, can you tell me more?
     
  5. Jul 28, 2013 #4

    ZapperZ

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    ... and how do you propose to achieve that?

    And how much air do you think you can "suck out", i.e. what vacuum level do you think is sufficient? Do you think then that the vessel that you put these blocks in are also electrically neutral, devoid of stray fields, etc? I used to do photoemission spectroscopy, and we go to great lengths to line our stainless steel vessel with mu metal just to get rid of stray, minute, magnetic fields that most people don't even care about!

    Zz.
     
  6. Jul 28, 2013 #5

    jtbell

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    You don't have to look as far as the sophisticated experiments that ZapperZ referred to, in order to observe gravitational attraction between laboratory-sized objects. Henry Cavendish first studied this experimentally in the late 1700s:

    http://en.wikipedia.org/wiki/Cavendish_experiment

    A version of his experiment is common in intermediate-level undergraduate physics laboratory courses:

    http://www.oberlin.edu/physics/catalog/demonstrations/mech/cavendish.html
     
  7. Jul 28, 2013 #6
    I could only imagine, but those effects would simply slow down the rate at which the two object converge I assume? In my case, getting u of friction to be 0 of course would be impossible I assume under the laws of physics, so the friction force would always be uN= umg. The forces would look something like
    -umg + m*m*G/r^2 = ma, losing the m we get -ug + m*G/r^2=a , and so since we calculated m*G/r^2 to be 6.67*10^-5 m/s^2, then if u max= 6.67*10^-5/g= 6.6*10^-6, the acceleration would be 0 which means they won't move at all.

    Now I see how that little friction between two surfaces is probably impossible. Am I on to something? How does the torsion experiment work by the way?
     
  8. Jul 28, 2013 #7
  9. Jul 28, 2013 #8
  10. Jul 28, 2013 #9

    Drakkith

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    That was pretty cool. Didn't know you could test gravity like that in your basement.
     
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