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Gravity, forces, and feeling your weight

  1. Aug 10, 2008 #1
    Hi there! I was wondering if you folks could give me a little enlightenment.

    Two days ago, I read a quote which led Einstein to a couple of brilliant discoveries about not being able to discern a force and gravity; if you're inside a box. The quote said that Einstein was suddenly struck by the thought that a person in free-fall, does not feel his own weight.

    I fully agree with that thought. But when do you feel your weight? Is it when a force acts on a object which is already being "forced" in the opposite direction? Or is it merely when you experience an acceleration?
  2. jcsd
  3. Aug 10, 2008 #2


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    You "feel" your weight when there is a "counter force" acting usually on your feet. In this case, we call this a normal force acting "upwards".

    If you are in an elevator in a free fall, everything is falling at the same rate at any instant - you, the elevator floor, etc. So the elevator floor is not pushing back up on you. Nothing is. You no longer feel your weight. So it is this counter, normal force that gives you your feel of your weight.

  4. Aug 10, 2008 #3
    Aha ;) Thanks! (And I am familiar with Normal force, etc.)

    So, could you say that you feel a force because your body is being stretched or compressed? Well, dunno if you understood my point with that one, but in any case, I feel that I understand Einstein's point. Brilliant ;)
  5. Aug 10, 2008 #4

    D H

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    You can never feel your own weight. You can, however, feel your apparent weight. Weight is tautologically defined as mass times the acceleration due to gravity; i.e., the gravitational force acting on a body. Apparent weight is the net sum of all forces acting on a body except for gravity. If you are standing on the surface of the Earth the two are nearly equal but opposite. (The rotation of the Earth makes your apparent weight a tiny bit smaller than your actual weight.)

    To the pilot of fighter jet pulling out of a steep dive, there is a large difference between actual and apparent weight. The pilot's actual weight doesn't change, but his apparent weight can be several times your actual weight.
  6. Aug 10, 2008 #5
    This is not the cause of feeling our weight, but because the strength of a gravitational field will vary somewhat over any spatially extended body, all macroscopic objects are stretched by gravitational tidal forces. This is a seperate issue from feeling our own weight, though.

    The reason you feel your weight is because there is a chair or floor in the way of your natural motion, to follow a geodesic in a curved spacetime.
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