Gravity - Einstein vs Newton

  1. Hi my background is in mechanical engineering. I use a little bit of motion physics but not often.
    Pls excuse any errors or wrong terminology I may use.

    Einstein's theory of relativity addresses the force of gravity in space time but does it address or apply to gravity on Earth? What creates or causes the gravity on earth? Gravity which is responsible for holding everything to the surface of the earth. From objects at rest or in motion to liquids and solids.

    I believe Newton states that mass creates gravity. The greater the mass the greater the gravity. Isaacs equations are still widely used today in engineering. I have often referred to them myself.

    So if I put a bowling ball in a vacuum of space will a small ball bearing be drawn to it due to a gravitational force?
     
    Last edited: Apr 18, 2014
  2. jcsd
  3. Meir Achuz

    Meir Achuz 2,061
    Science Advisor
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    Yes.............
     
  4. What a genius Isaac Newton really was. He blows my mind

    So has this practical example been performed and confirmed?
     
  5. Look up the cavendish experiment, that may be interesting to you.
     
  6. Excellent. I believe the experiment was done under earths gravity.
    Have other experiments been performed in vacuum enclosure?
    I would like to see a huge suspended mass with a tiny object pinned to the bottom by gravity. To simulate myself standing upside down in Australia
     
    Last edited: Apr 18, 2014
  7. phinds

    phinds 9,369
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    Newton's equations are what fall out of GR in a limiting case, so there is no difference between the two, really. Anything that Newton's Law of Gravity predicts is also predicted by GR, just more correctly and also in a way that works for the more general case. But it IS way easier to use Newton, so folks who build bridges and such don't use GR because they don't have to (they are working inside the limited case).
     
  8. It was done near the earth, but it was not testing the gravity from the earth. It was testing the gravitational attraction from one ball to the other perpendicular to the direction the earth would attract.

    Even in the vacuum of space there is gravity from earth.
     
  9. A.T.

    A.T. 6,281
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    Yes. This animation has almost the same title as your post:

    https://www.youtube.com/watch?v=DdC0QN6f3G4
     
  10. Thanks to everyone for the feed back.

    @ModsPwnd. I understand their testing gravitational forces of bodies. Not the earths gravity.
    I just imagine putting a large mass which represents earth in a vacuum chamber. Test its gravitational pull on a small object representing a car. No strings attached. Proportionate in scale. That would be cool to see. A scaled down example of the gravitation attraction of earth
     
  11. @A.T. Very interesting animation. So GR does in fact apply in earths atmosphere also. Thank you for answering one of my questions. Lol. What a freak of nature Albert Einstein. So much perseverance the man had also.

    I shouldn't have titled this Einstein vs Newton. I wanted to compare the two. That sounds like boxing match which was never my intention
     
  12. @Phinds What do u mean by "a limited case"
    I agree Newtons equations of gravity and motion are much easier and more applicable to everyday science and engineering on earth. His equations are still being used everywhere in everyday applications.

    GR is much harder to grasp and is really used in astrophysics, true? The precision of Einstein's field equations over great distances into space is just phenomenal.

    Thanks again guys for helping me understand a few things.

    I'd say these two men are pure geniuses. Ahead of their time. The biggest contributors to an amazing world called Physics
     
  13. phinds

    phinds 9,369
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    There are satellite in orbit that are smaller than a car, so this is being demonstrated every day. There is no difference between that and what I think you mean which is to put them both very far removed from other gravitational forces and have the start off stationary relative to each other an watch them come together. That wouldn't prove anything not already shown by current satellites in orbit.

    Newton's Law is just GR for small masses traveling slowly.
     
  14. Ah yes I was going to state that there wouldn't be anything to learn really. It would just b a cool scaled down experiment to observe I feel.
    GR is definitely for larger bodies at greater speeds over larger distances
     
  15. phinds

    phinds 9,369
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    Well, not really LARGE bodies but more massive bodies (which, I realize, does usually come with "larger" but lets get the terminology right) and not over larger distances. Newton works just fine over large distances. Stick with "more massive" and "moving faster" (LOTS faster :smile:)

    EDIT: actually "more massive" isn't right either. More DENSE is what it is. Newton works just fine without mass limitation, it just doesn't work in very high gravity fields such as near an extremely dense object like a neutron star or a black hole.
     
  16. I did mention in my opening paragraph to excuse any incorrect terminology.
    I know what ur saying.
    Thanks again to all
     
  17. Newton: A falling body has a downward gravitational force acting on it, and it is accelerating.
    Einstein: A falling body has no force acting on it, and it is not accelerating.

    Newton: A body at rest on the earth has no net force acting on it, and it is not accelerating.
    Einstein: A body at rest on the earth has an upward force acting on it, and it is accelerating.

    Einstein's description is a consequence of the curvature of spacetime and the movement of bodies along their (curved) world lines. The presence of a massive body like the earth causes the region of spacetime in its proximity to become curved.

    Chet
     
  18. Acceleration is the time derivative of velocity.

    It sounds incorrect to me to say that a body in freefall is not accelerating. It makes me think I can step off the top of a tall building and everything will be alright because I won’t accelerate.
     
  19. In the context of general relativity, what you are referring to here is called "coordinate acceleration", but it is not true acceleration. You know that it is not true acceleration because, if you had an accelerometer attached to the body, the accelerometer would read zero. This has been verified experimentally. When you are in free fall, all you are doing is traveling along the geodesics of curved spacetime.

    Chet
     
  20. phinds

    phinds 9,369
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    Yes it does sound weird, but you want to watch out for that. Under Newton, You'll die because of acceleration and under GR you'll die because of the curvature of space-time. That's probably a nicety you won't care much about on the way down. :smile:
     
  21. Its not the acceleration due to gravity that kills you... Its the acceleration due to the coulomb force at the pavement. You can accelerate at any rate and be just fine. Thats because all your atoms get accelerated at the same rate (as long as the tidal force is low).
     
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