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A Gravity Question

  1. Feb 20, 2010 #1
    Hello people

    What is the difference between the person's reaction to gravity and the normal upward force acting on the person on the surface which he's standing on (ground reaction) ?

    i think ground reaction is due the the person standing on the ground who exerts a force on it due to gravity (person's weight) but its diffrent from the person's reaction to gravity although they have the same magnitude.while the reaction of the person is the person attracting earth towards him right?
  2. jcsd
  3. Feb 20, 2010 #2
    Yes. The Newton 3 reaction to the gravitational force is the attractive force that the person is applying to the Earth.
    The normal reaction of the surface is the upward force that it exerts on the person, and stops them from falling through the ground. The Newton 3 reaction to that force is the force of the person on the Earth.
    As far as the person is concerned there are two forces. The Earth pulling downwards (mg) and the surface of the Earth pushing upwards. This force is numerically equal to mg if the person is not accelerating upwards or downwards.
  4. Feb 22, 2010 #3
    Yeah i can imagine this
    Ok but why don't we say that the body's reaction is responisble for stopping the body from falling to the ground instead of the normal upward force??

    also why does buoyant force make weighlessness?
    Thanks very much
  5. Feb 22, 2010 #4


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    Perhaps you are confusing Newton's 2nd and 3rd laws. Newton's 2nd law regards the influence of forces on a particular object. Newton's third law regards the influence of a body on its environment (e.g. a second body) due to the influence of the environment (e.g. second body) on the (first) body.

    In the present discussion:

    Newton's 2nd law regards two forces acting on one body: gravity and normal force acting on the body.

    Newton's 3rd law regards two interaction forces between two bodies: their gravitational attraction and their normal force repulsion.

    It doesn't. Where did you hear this? Buoyant force provides a force in the opposite direction to gravitational force. In terms of the present discussion, buoyant force is the same as normal force in terms of how it acts on the body (pushing it upward against gravity), and the role that it plays in the reaction (repulsion between the two gravitating bodies).
  6. Feb 22, 2010 #5
  7. Feb 22, 2010 #6


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    It is true that astronauts are trained in water for working in the "weightlessness" of near-Earth orbit. However, this does not give them a feeling of orbit-like weightlessness, because all of the free tissue in their body (in particular endolymph) still experiences a downward influence due to gravity (unlike in gyrostatic orbit where gravitational and centripetal acceleration almost exactly match).

    The desired "feeling of weightlessness" that they experience in the water is just a psychological interpretation of neutral buoyancy, which is essentially a freedom to move their body in space without the requirement of a fixed support force. The key difference between this kind of feeling, and the "true" weighlessness experienced in orbit can be revealed by simply closing their eyes. If they can determine, by intrinsic physiological sensation, which direction is "down", then they are not in orbit. Rather, an intrinsic orbit-like sensation is acheived in the "vomit comet" (or the initial stage of any kind of "free-fall")
    Last edited: Feb 22, 2010
  8. Feb 24, 2010 #7
    fixed support force like what?
  9. Feb 24, 2010 #8


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    Please rephrase your question.
  10. Feb 24, 2010 #9
    Would they be an exact match, assuming this centripetal acceleration is towards the centre of mass of the earth, if and only if

    (1) there were no other bodies to perturb the system;

    (2) the satellite was a test particle, i.e. of negligible mass and extent;

    (3) the earth was perfectly spherical?
  11. Feb 24, 2010 #10
    If a mad global dictator were to order everyone on the earth to run eastwards all at the same time, would the earth's orbit speed up? Just something to think about... :biggrin:
    Last edited: Feb 24, 2010
  12. Feb 24, 2010 #11


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    I suppose, if there were also no atmosphere.
  13. Feb 25, 2010 #12
    It would certainly give those living on the East Coast a bit of a headache.
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