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A good way to look at gravity?

  1. Nov 2, 2006 #1
    Hey guys. I know what gravity is and everything, but I was wondering if this would be a good way to look at gravity in a physical sense (even though you can't nessesarily "see" gravity, but observe it's effects.)

    Lets say you have a planet, or star. Since there is a lot of mass in the planet/star or whatever, the planet bends space. Would this be a good analogy since gravity attracts?

    I find it very strange how gravity attracts objects. It seems somewhat logical to think if an object is in space, it is pressing "down" on the "fabric" of space. The downward push causes gravity, therefore attraction to the object with suficient mass.

    I guess a good example would be this: Let's say you have a bed sheet that represents space/time and a baseball that represents a planet. Now if you stretch the sheet out somewhat tightly with 2 people and place the ball on the sheet, the ball causes the sheet to bend downwards a little bit. Now if you have let's say a marble, and place it on the sheet, if it's close enough, it will "attract" (or since gravity exists on earth, travel downward to the lowest possible point.) However if it's not close enough to the funnel, it stands still, and also causes a small amount of indention to the sheet itself. I know this analogy is flawd in the sense it's on earth, but I'm just trying to generalize and make sense of it!!!

    I'm familiar with Newton's laws and relativity and all, I just want to know a good way to picture it in my head. Please try to verify this analogy or enlighten me! Thanks guys
    Last edited: Nov 2, 2006
  2. jcsd
  3. Nov 2, 2006 #2


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    That's pretty much the textbook "balls on a rubber sheet" model used for GR, yep.

    (In fact - don't take this the wrong way - it sort of indicates that you haven't done a lot of reading on the subject, since you would have stumbled over the rubber sheet model immediately.) You pretty much nailed it.
    Last edited: Nov 2, 2006
  4. Nov 3, 2006 #3
    You're right lol. I've been out of highschool for 4 years and I'm temporarily out of college... The last science last I had dealing with gravity probably was my freshmen year of highschool... Didnt' take physics in highschool but going for astronomy/physics in 2 years at LSU !! Can't wait!
  5. Nov 3, 2006 #4
    What level is your mathematics?
  6. Nov 3, 2006 #5
    I have good math skills. I am fully aware of the advanced math involved with physics.
  7. Nov 4, 2006 #6
    The rubber sheet analogy is somewhat misleading because it's actually the curvature of time that usually dominates. For light grazing the sun, the curvature of space accounts for about half the deflection. See Schutz, Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity, Ch. 18.
    Last edited: Nov 4, 2006
  8. Nov 13, 2006 #7
    that analogy is very similar to the one used by Michio Kaku in parallel worlds :wink:
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