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How does normal force makes us feel our weight?

  1. Feb 5, 2016 #1
    So I have been asking my self these questions lately. I know it is some basics of physics and I should probably know the answer..

    Why do we feel weightlessness when we hit the acceleration of gravity (9.8) exactly?
    Probably so that the surface that we are standing on doesnt need to produce any normal force on us so we dont feel weight. For instance, The elevator.. When the acceleration is exactly at 9.8 downward
    then Mg - n = ma
    and by that n = 0

    But how do normal force makes us feel our weight? that what I couldnt understand.
     
  2. jcsd
  3. Feb 5, 2016 #2

    A.T.

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    What you feel are strain and compression of your body. For that you need a non uniform force, like a contact force acting only on some body parts. A uniform force field, like gravity on small scale, cannot cause such strains.
     
  4. Feb 5, 2016 #3
    I think I have got what you mean. Yea awesome :)
    but one question, Gravity cant cause strains because gravity affects each part of our body with the same acceleration right?
     
  5. Feb 5, 2016 #4

    A.T.

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    Yes, unless you have a strong gravity gradient.
     
  6. Feb 5, 2016 #5

    russ_watters

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    My feet hurt.
     
  7. Feb 5, 2016 #6

    Drakkith

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    It can and does cause strains, but on Earth at our everyday scale the gradient isn't strong enough to notice, hence why A.T. answered "No" to your question. If we increase the gravity or increase the scale then the gradient's effects become noticeable. For example, the gravitational gradient of the moon's gravity on Earth is large enough to cause tides. See here for more info: https://en.wikipedia.org/wiki/Tidal_force
     
  8. Feb 6, 2016 #7
    That's true
    The gravitational force acting on us is not strong enough to cause strains/compressions in our body
    As a result it's the reaction force of the surface or rather the normal force which causes us to feel our "weight"
    Imagine you were falling freely
    And don't neglect air resistance
    Wouldn't you have some deformation in your face/mouth?
    What force do you think causes this deformation(it's a type of frictional/drag force exerted by the air)?:)



    UchihaClan13
     
  9. Feb 6, 2016 #8

    HallsofIvy

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    We don't. We feel "weightlessness" when there is no floor beneath us to press upward on us. The acceleration is irrelevant.

    "Normal force" (specifically force normal to the floor) is the definition of "weight".
     
  10. Feb 6, 2016 #9
    The acceleration is relevant in Earth daily life I believe. If you are going down in elevator in a acceleration of lets say 3 m/s/s and you jump you will be dragged down because your speed is much faster than the elevator( because the acceleration of gravity) so I guess from what you are saying that we feel it for a few seconds because we are not touching the ground?
    But in a elevator with 9.8m/s/s acceleration and we jump then ( I think that we will hit the top of the elevator because the speed of it will be always faster than ours until we get the same speed) But we wont reach the bottom because it is moving away from us at the same speed we are moving to it.

    I guess I am blabbing too much right? XD
     
  11. Feb 6, 2016 #10

    HallsofIvy

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    No! You are not "dragged down by the elevator. You are "dragged down" by the force of gravity. If you jump up in an elevator, as long as you are not making contact with the floor of the elevator, it is exactly the same as if the elevator were not there. Of course, you will have, at the instant you jump, the same speed as the elevator. As long as the acceleration, downward, is less than the acceleration due to gravity, you will shortly make contact with the floor. If the acceleration of the elevator itself were -9.81 m/s^2 the elevator itself would be in "free fall" as would any one in it. Again, it would be exactly as if the elevator were not there. (I hope you are not confusing "acceleration" and "speed". For the great majority of any elevator ride, up or down, the elevator's acceleration is 0.)
     
  12. Feb 6, 2016 #11
    ...
    When did I said dragged down by the elevator ._.?
    What you said is what I said above :/.
    " you jump you will be dragged down because your speed is much faster than the elevator( because the acceleration of gravity)"
    Well I didnt mean dragged down by the elevator. It is just because your speed becomes faster than the elevator so you reach its floor. Maybe dragged wasnt the best word to use but you can say fall down to the floor it is just an expression
     
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