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3rd Law Question

  1. Jan 13, 2016 #1
    Ok I always understood the 3rd law and normal force well until recently when I started overthinking it. In an elevator, your weight pushes down on say a scale. That scale pushes back onto you with an equal and opposite force. But if the elevator is accelerating upwards, then do you exert an even greater force on the scale and the scale in turn exerts an even greater force back on you? How does the 3rd law apply in an accelerating elevator?
     
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  3. Jan 13, 2016 #2

    DrClaude

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    Staff: Mentor

    Yes, the same way that you press harder against the seat in an accelerating vehicle.
     
  4. Jan 13, 2016 #3

    A.T.

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    No, "your weight" pulls down on you. The 3rd Law opposite force to this is you pulling on the Earth.

    That is another 3rd Law pair. Both are contact forces. None of them is weight. And they don't have to be equal to your weight.
     
  5. Jan 13, 2016 #4
    So then how is there a net force to force your body to accelerate in that situation?
     
  6. Jan 13, 2016 #5

    jbriggs444

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    The force of gravity on your body is not equal to the force of the elevator on your body. The net is non-zero.
     
  7. Jan 13, 2016 #6
    Ok, so there is a net force upward because your weight is not as large as the the force of the scale on you. The force you exert on the scale as part of the 3rd law doesn't really matter right?
     
  8. Jan 13, 2016 #7

    jbriggs444

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    Right!

    The force you exert on the scale counts toward the net force on the scale, not toward the net force on you.
     
  9. Jan 13, 2016 #8

    sophiecentaur

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    Do not confuse the Third Law with notions of equilirium and the First Law. The third law is to do with the interaction between two objects and the conservation of Momentum. The First Law is to do with what happens to an object when subjected to one of more forces.
     
  10. Jan 13, 2016 #9
    I see. Like I said I was overthinking, and the last thing I need to clear up is if you exert a force on that scale, what is stopping that scale from accelerating downwards? The force of the elevator floor? And the elevator is accelerating most likely through tension so that's why the scale doesn't move downwards right?
     
  11. Jan 13, 2016 #10

    sophiecentaur

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    Any downward force on the floor could affect the speed of ascent or descent but the whole machine is very inefficient (lots of friction) and the effect could be undetectable.
     
  12. Jan 13, 2016 #11
    Alright thank you all!
     
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