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Gravity's Acceleration and its Effect

  1. Jun 11, 2013 #1
    So, one of Newton's Laws of Motion is -F(sub)1 = F(sub)2.

    Gravity is (generally) pulling us down with an acceleration of -9.8 m/s^2 where negative is downwards. Because of the above law, it has been said that this is felt upwards in objects. Why do I only feel gravity downwards?
  2. jcsd
  3. Jun 11, 2013 #2
    Those forces act on different objects. Gravity pulls you down with some force; you pull the earth up with the same force.
  4. Jun 11, 2013 #3


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    It is not felt upwards because of Newton's 3rd Law. When you are standing on level ground, there is a normal force from the ground on you which points upwards and tends to cancel the gravitational force, due to the Earth, pointing downwards from your center of mass. When you are walking on the surface of the earth, the ground pushes up on you and you will of course feel this; the normal force in such a scenario just happens to cancel out the force of gravity pulling you down, allowing you to stay on the level ground. This normal force however is not a reaction force due to the gravitational force exerted by the Earth on you! The reaction force of the Earth's gravity pulling on you is simply your gravitational force pulling back on the Earth.

    As stated above, it is true that what you feel is the upwards pointing normal force from the ground on you. In fact, what Einstein said is that if you were completely sealed in a small elevator, you would have no way of telling if you and the elevator were at rest in a downwards pointing uniform gravitational field of magnitude ##g## or you and the elevator were accelerating upwards with magnitude ##g## in free space. This upwards acceleration of magnitude ##g## is what you feel. In the case of a person standing on the surface of the Earth, you feel the upwards normal force, since the ground is pushing up on you.
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