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Gravity = ?

  1. Jul 13, 2003 #1
    I have a good idea of the basics of gravity.

    But I have one question...

    What determines the strength of an object's gravity? Just mass?
    Or does density determine the strength of the gravitation pull?

    i have a follow-up question or 2 that depends on what the answer to the above question is, so i'll ask that once my above question gets answered.
  2. jcsd
  3. Jul 13, 2003 #2
    The strength of gravity is reliant of an objects mass. The density of an object is derived from the equation D=m/V and so density simply means that a dense object can be smaller than an undense object and have the same gravitational pull.
  4. Jul 13, 2003 #3
    ok. i'm going to continue asking questions regarding gravity in this thread...

    does each particle have a gravitational pull? or, rather, does the object as a whole?

    also, when a particle is part of an object, that particle contributes to the object's gravitational pull, correct? AND that particle also has its own, personal gravtitational pull on the nearby particles, at the same time?

    if each particle has its own gravity (a weak one, obviously), how does one form a "massive object" that has a strong gravitational pull---just by simply being in the vicinity of a bunch of particles, you have a strong gravitational pull?


    what are the possible mechanisms for gravity (how exactly does it work?)?

    as far as I know, the answer to this question is not known for certain.

    the 2 possible mechanisms that I can think of are:

    - distorting space-time (i.e. ball on sheet of rubber, bending the surroundings)
    - gravitons (-imaginary- particles)
    - mix of the above 2?

    one little thought that just came to me: let's say you are looking at a plane. Got this in your head? A 2D plane in 3D.

    Now, this 2D plane represents a collection of particles, which all lie on the plane (say, in a rough circle). There are "dimples" and "bumps" in the plane that represent the strength of the gravitational pull of each, individual particle. 1 billion particles, 1 billion dimples.

    So, each particle has its own gravitational pull. Each particle has a slight pull on the surrounding particles.

    With me so far?

    Ok, so how does this collection of particles have 1 big gravitational pull? It just has a billion "little pulls" -- a bunch of little dimples, not one big huge dip.

    And yet, in real life, this collection of particles --aka object-- has a (relatively) large gravitational pull. But -- if you look at the object as a collection of individual particles, all you see is many many dimples!!

    note: (in real life, it'd actually be a billion billion billion billion ... billion particles, not the "1 billion" that I used for brevity)

    So how does this collection of particles, each with their own mass and thus, gravitational pull, suddenly have one, large, collective gravitational pull?
  5. Jul 13, 2003 #4


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    Staff: Mentor

    Each individual particle has its own pull, but to simplify calculations, we deal mostly with full objects.
    Pretty much.
    A lotta little forces added together equals one big force.
    Ok, this really shouldn't be that hard. Imagine you had 100 rubber bands that each exerted a 1 lb force on an object. What's the total force?

    Or rather, in your dimples analogy, if the dimples are really close together, you start getting dimples on top of dimples, making them combine to form one big one. Have you ever had poison ivy...?
  6. Jul 14, 2003 #5


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    Science Advisor

    Have you ever actually done this? That is, take a number of small masses: shot, say, and place it on a rubber sheet.

    Each shot makes a little dimple, yes, but when two are close together, you DON'T have the two dimples coming up level with the rest of the sheet between them: what happens is you get one, larger, dimple. All of the "dimples" (distortions in space-time) combine to form one large dimple.
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