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Gravitation force on Earth

  1. Jul 2, 2010 #1
    HI,
    According to GR, gravitation is simply the geometry bending. Does this also apply on Earth? We can stick on Earth, does it the force attract us?? Or is a geometry problem as well??

    I get confused about this.

    Thanks
    Alex
     
  2. jcsd
  3. Jul 2, 2010 #2

    Mentz114

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    Regardless of what any theory says - gravity is a force - you can feel it. It just so happens that the 'charge' ( gravitational mass) is the same as the resistance to movement ( inertial mass) so you can make a mathematical model in which the force becomes geometry.
     
  4. Jul 2, 2010 #3
    Thanks!
    So, the light bending (near Sun) is due to force basically.

    One more question, do we also suffer from gravitation wave on Earth as well??
    Alex
     
  5. Jul 2, 2010 #4

    Mentz114

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    Remember that Newtonian gravity also predicts light bending by the sun.
    I suppose if gravitational radiation is out there, we are exposed to it.

    There's a good discussion in the concurrent thread 'Is gravity an inertial force ?'.
     
  6. Jul 2, 2010 #5

    D H

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    I disagree. You don't feel gravity. What you feel when standing on the surface of the Earth is the normal force propagating through your body. Suppose you take one of those amusement parks rides that take a sudden 1g drop. Right before the bottom falls out on the ride you don't feel anything different than you did 100 meters or so down when you were standing on the solid Earth. When the ride lets loose you no longer feel that the normal force propagating through your body. There is no change in gravitational force, but your equilibrioceptors nonetheless are telling you that something is very, very wrong.

    The same goes for accelerometers, and gravimeters, and any other local experiment that purportedly measures gravitation. Those sensors measure everything but gravity. The way to explain this in Newtonian mechanics terms is to mystically invoke gravitation as the only real force that cannot be sensed. The general relativistic explanation is much easier: gravitation is not a real force. Of course you can't sense it.
     
  7. Jul 2, 2010 #6
    If gravity is a force or not is a matter of semantics.

    Of course we can sense it otherwise there would be nothing to measure. Masses attracting, bodies in orbit, time dilation, light bending. There are more things in nature to be sensed than acceleration.

    Also, free falling (or floating) objects will experience, albeit extremely small in most cases, tidal forces.
     
    Last edited: Jul 2, 2010
  8. Jul 2, 2010 #7

    Mentz114

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    I knew someone would. What name would you give the normal force ? I know some forces can be transformed away, but the frame in which it's gone, is itself moving under the influence of ... gravity.

    You are inviting the question "what do you mean by 'real' ?"

    Agree. The acceleration in the direction of the tangent vector is nothing for a geodesic, but there are still effects orthogonal to the tangent plane.
     
    Last edited: Jul 2, 2010
  9. Jul 3, 2010 #8
    there is a basic law mentz.....
    any motion is due to a force....
    irrespective of the force on the force....
    there is my Earth moving round the sun due to gravitational force....
    now consider a pendulum covering 360 degrees in one cycle....
    the thread here is the motion controller n ultimately the force controller
    But still the Earth follows a projectile motion around the Sun... And the pendulum follows rotational motion....
    And please a projectile on Earth is also in an infinite frame of ref. still only a force is making it possible....
    Gravity is a force.... Saying that its a geometrical drama of the space time fabric wud suffice only the definition of force......
     
  10. Jul 3, 2010 #9

    D H

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    Depending on who you talk to, the normal force is a consequence of electrostatic repulsion or the exchange interaction. Certainly not gravity. Suppose you push against a wall. The wall exerts a horizontal normal force on you. That is the normal force.

    The opposite of fictitious.
     
  11. Jul 3, 2010 #10


    Although I know this is common explanation given when talking about gravity as force or not, I still have big issue with it.

    How can any instrument register any force in free fall when every and each part of it, is subjected to the same acceleration (I know that you will object here that things in free fall are not accelerating, but let's stick to newtonian view). It simply can't.

    Same goes for humans. You can't feel anything if every part of you is accelerated evenly.

    So because gravity influence everything proportional to the mass, and that things end up evenly accelerated (F=ma) does not mean that it is not a force.
     
  12. Jul 3, 2010 #11
    I suggest you try out your theory in an elevator. Tell me if you can feel the push when you are accelerated upwards.
     
  13. Jul 3, 2010 #12
    Hi But in an elevator every part of you is not accelerated evenly. The acceleration starts at your feet and propagates through your body from there. It is the inertial resistance to this propagating added momentum that causes the "feeling" of force.
    No?
     
  14. Jul 3, 2010 #13
    In elevator you can feel push, because elevator is exerting force on your legs which is then transfered through your body (organs, blood, bones, etc..). When you are accelerating due to the gravity you don't feel a thing, because every part of you is accelerated evenly.
     
  15. Jul 3, 2010 #14
    Profound agreement on the semantics.

    On sensing and measuring it . Couldn't you say that you are actually measuring the effects?
    IN principle you can measure all the effects of inertial motion; dilation, contraction etc.
    but cannot sense them within a system and we consider them a result of spacetime geometry with no force involved and so unreal.

    Are there actual experimental observations of tidal forces?
     
  16. Jul 3, 2010 #15
    Exactly. You feel a force when you are accelerated. Therefore, you don't accelerate when you are in free fall. The normal force you feel when standing on Earth is there because you are being accelerated.
     
  17. Jul 3, 2010 #16
    Are you suggesting that the force normal to the ground that you feel standing is due to electrostatic exchange or interaction?
    As apposed to the individual particles of your body being in inertial motion on geodesics and meeting resistance from the ground?? This resistance to inertia propagates through your body succeeding altitudes inertially pushing on the proceeding and this is what you feel?
    Still a fictitious force of course . But then isn't the normal force exerted by the wall a totally fictitious force??
     
  18. Jul 3, 2010 #17

    Doc Al

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    :confused: Why would you think that the normal force is fictitious?
     
  19. Jul 3, 2010 #18
    This exemplifies a strange confusion and mixup of Newton and GR within the forum.

    Newton------- G is a force ----> -r

    GR -------- G is not a force but a geometry.

    GR --------- G is a force but -----> +r
     
  20. Jul 3, 2010 #19
    That is simply not truth. You feel force when you are accelerated by mechanical means.

    And you are not accelerated when you are standing on earth, because it would mean that work is being done on you, and that some energy is being transfered. Work is force times distance, and since distance is zero, work is also zero.
     
  21. Jul 3, 2010 #20
    In GR, gravity is a ficticious force, or psuedo-force. The gravitational field can be made to vanish by a choice of coordinates. In an accelerating reference frame, like that of the surface of the Earth, gravity accelerates things and is a force. But in an intertial or locally inertial frame, gravity does not accelerate things and is not a force. Therefore, it is on equal standing as the centrifugal force.

    Distance is a coordinate dependent quantity, and so is work.
     
  22. Jul 3, 2010 #21
    Because the force involved is being exerted by your muscles. The force you feel is meeting the constraint of the wall .

    Isn't this equivalent to the fictitious centripetal force encountered by particles accelerated by the fictitious centrifugal force in a centrifuge when they meet the constraint of the wall of the machine??
    So isn't the wall equivalent to the bounds of the centrifuge?

    I have a question: COuldnt you look at particle collisions and accelerations as transfers of momentum with conservation but without any need for a concept of force?
    As inertial paths with only relative coordinate changes in velocity and direction.
    Equivalent to a particle in inertial motion in curved spacetime with only relative coordinate changes in velocity??
    The concept of Force would then be a useful bookkeeping procedure but essentially fictitious . The same as Newtonian gravity, centrifugal etc.
    Thanks
     
  23. Jul 3, 2010 #22

    Doc Al

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    Sounds pretty real and non-fictitious to me! The normal force is a contact force between two surfaces--very real in any frame.

    The centripetal force is quite real. (The centrifugal force is a fictitious artifact of viewing things from a rotating frame.)
     
  24. Jul 3, 2010 #23
     
  25. Jul 3, 2010 #24
    The surface of the Earth is indeed accelerating in space-time. Why? Because it deviates from geodesic motion. Free fall, that is.

    Newton's first law states that when there are no forces on an object, it travels in a straight line. This is exactly what is meant when one says gravity is a geometric artifact. Gravity is not a force, so objects in free fall move in straight lines through space-time. Mass and energy curves space-time, so straight lines in space-time appear curved in space.
     
  26. Jul 3, 2010 #25
    I understand what you are saying but simple contact between surfaces does not involve force. It is only when momentum is added to one surface or the other that there is resistance. I understand the third law and that in priciple the force is equal and opposite but in actual practice in this example the muscles provide the momentum and the wall provides an overwelming inertia. Just as in the accelerating elevator the motor provides the muscle and momentum and we as passengers provide inertia which we feel as force.
    Isn't what is experienced in both cases ficticious?


    In a centrifuge of course the bounding walls are real as is the momentum imparted to the contents but is the force any more real than in the elevator?
    The centripetal force of Newtonian G acting against inertia to hold the planets in orbit is now viewed as ficticious. No?
     
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