Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravitational attraction

  1. Jul 22, 2009 #1
    Suppose that in free space , dust particles are evenly distributed all around.
    Amidst the dust particles , there are two identical spherical cavities at some distance from each other. Do you think they will attract each other or repel or just be as they were??
     
  2. jcsd
  3. Jul 22, 2009 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    They repel. (Wrong....see below)
     
    Last edited: Jul 22, 2009
  4. Jul 22, 2009 #3
    And may i know how? I always think that they would attract each other as the mass around the spheres would try and close on in , i was thinking of going about this by using the principle of superposition but still couldnt get a conclusion.
     
  5. Jul 22, 2009 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member

  6. Jul 22, 2009 #5

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Oops....sign error.

    Hold one bubble fixed. Consider the force on the second bubble. If the first bubble didn't exist, there would be no force on the second. Since there is a gap, the gravitational force points away from the bubble. So the dust moves in that direction, away from the bubble, which means that the void moves towards it.
     
  7. Jul 22, 2009 #6
    Hey thanks that cleared a lot up.
     
  8. Jul 22, 2009 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Another way you can think of it is to treat the dust as massless, and the bubbles as having negative mass. The force points in the opposite direction, so the acceleration points in the same direction: the bubbles attract.
     
  9. Jul 22, 2009 #8

    DaveC426913

    User Avatar
    Gold Member

    What are these cavities composed of? Are they simply voids in the distribution of the particles? Is there anything holding them rigid? Is there anything holding the particles from filling them?
     
  10. Jul 22, 2009 #9
    Yes , they are simply voids. The Dust particles are distributed uniformly and there are two nearby spherical voids.
     
  11. Jul 22, 2009 #10

    DaveC426913

    User Avatar
    Gold Member

    Then it is folly to pretend they act as an object. The particles will uniformaly distribute themselves within them.
     
  12. Jul 22, 2009 #11

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Even if all particles are initially at rest?
     
  13. Jul 22, 2009 #12

    Danger

    User Avatar
    Gold Member

    Won't all of the dust particles attract each other into a assemblage, thus merging the voids into one huge one that surrounds the object? (No offense, Vanadium, but the original question didn't assume the dust to be massless.)
     
  14. Jul 22, 2009 #13

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Yeah, I guess dust particles with empty space between are quite different from a fluid, which cannot be compressed or vaporize. How would they behave with only one spherical cavity, when initially all are at rest?
     
  15. Jul 22, 2009 #14

    DaveC426913

    User Avatar
    Gold Member

    They cannot be at rest; they must have some thermal energy.
     
  16. Jul 23, 2009 #15

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Why?
     
  17. Jul 23, 2009 #16
    Because if they lacked thermal energy, and they were at rest, you would pretty much know everything about them with certainty (position, velocity, energy) which violates the uncertainty principle
     
  18. Jul 23, 2009 #17

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    I was thinking more in terms of larger particles so I omitted this. How would they behave classically without quantum effects, just gravity.
     
  19. Jul 23, 2009 #18

    DaveC426913

    User Avatar
    Gold Member

    OK, I see where you're going.

    The particles adjacent to the voids feel an imbalanced gravitational pull. The net effect is that they are pulled away from the voids, causing the voids to grow. The particles between the two voids have this imbalance ... uh ... balanced. They feel no pull towards or ways from the two voids, but they do feel a very small pull outward, as if being squeezed out from between the two voids.

    The net effect is that the voids will grow until they are separated by a thin filament of particles.


    If we expanded the model to include multiple voids in a 3D volume, we would see all voids grow until they were separated only by thin, dense filaments. Ultimately, the volume would be less "voids in a sea of particles" and more "empty space shot through with filaments", like a cobweb in a box.

    Interestingly, this is exactly what is http://images.google.com/images?q=g...oe=utf8&rlz=&um=1&ie=UTF-8&sa=N&hl=en&tab=wi".
     
    Last edited by a moderator: Apr 24, 2017
  20. Jul 23, 2009 #19
    Surface tension has a role?
     
  21. Jul 23, 2009 #20
    Hey! I just found out that its an ex Physics olympiad problem.
    The dust configuration remains constant , wont move in. So we can do this using the potential energy concept for the sphere's i.e. they would try and reduce their respective energies.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gravitational attraction
Loading...