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Everything attracts everything else; F = GMm/r^2. If two objects are

  1. Aug 4, 2012 #1
    Everything attracts everything else; F = GMm/r^2.
    If two objects are placed at an everyday-experience distance to each other and have everyday-experience masses on ice, where no other forces are exerted on the two-mass system besides the mutual gravitation attraction, will the two masses eventually bring each other closer together if given enough time?
     
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  3. Aug 4, 2012 #2
    Re: Gravitation

    If absolutely no other forces are involved? Yes.

    For everyday distances and masses, the forces are pretty tiny, and in a realistic setting, I doubt such a force could overcome even the force of static friction with the ice.
     
  4. Aug 5, 2012 #3

    Drakkith

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    Re: Gravitation

    Realistically, no, this would not happen because friction would keep them from moving. But if we could place two objects in a frictionless environment then yes, they would get closer together.
     
  5. Aug 5, 2012 #4
    Re: Gravitation

    How long would it take, really, for let's say, two 1 kg point masses separated at 1 m apart, to come into contact with each other (again, assuming they're both on frictionless ice).
    According to my calculations, which I doubt I've done correctly, I've got ~34 hours, which seems to be an extremely short amount of time when compared with intuition.
     
    Last edited: Aug 5, 2012
  6. Aug 5, 2012 #5

    Drakkith

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    Re: Gravitation

    Why does that seem short?
     
  7. Aug 5, 2012 #6
    Re: Gravitation

    The fact that gravity makes things come closer together is foreign to me, even if the concept of gravity is not, because we just don't see mundane things in everyday life do such a thing. So the idea that if you place two objects in a frictionless environment and it will come into contact with one another in just a matter of a day and some hours is just so strange.
     
  8. Aug 5, 2012 #7

    Drakkith

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    Re: Gravitation

    Well, I haven't done the numbers, but I can see it. Remember that gravity is constant, and it will continually accelerate the two objects towards each other. The fact that they are so small means it takes a very long time for them to get even the tiniest velocity. This contrasts to the Earth which will accelerate you to 9.8 m/s in just 1 second.
     
  9. Aug 6, 2012 #8
    Re: Gravitation

    We can skip the ice and just put them in free space. The space station or the now out of service shuttle would be excellent environments and one would think that they would have done it with two blobs of water.

    Another possibility would be to put two buoyant spheres in water. Surface tension would present a bit of an issue there, though.

    How else might we play this game?

    How about an air hockey table?
     
  10. Aug 6, 2012 #9
    Re: Gravitation

    Without paying careful attention (i.e. setting up a real experiment), you'll never see the gravitational attraction between everyday objects.

    One cool thing you can set up that can see this attraction is called a torsion balance. You can wikipedia it for details, but basically its a mechanism that translates the gravitational attraction between several bodies into rotational momentum of a rod: which is much easier to see.
     
  11. Aug 7, 2012 #10

    haruspex

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    Re: Gravitation

    I make the formula (π/4)√(d3/Gm), where d is the initial separation. That gives 26.7 hours.
     
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