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Elevator problem

  1. Sep 25, 2007 #1

    dnt

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    1. The problem statement, all variables and given/known data

    Quite simply, you are moving up at a constant speed in an elevator and you are to compare what a scale reading would be at this constant speed (given to be 700 N) to when you slow down to a stop.

    2. Relevant equations

    Fnet = ma

    3. The attempt at a solution

    Clearly you have weight (mg) downward and the Normal force upward. The normal force (up) is what the scale would read so thats what we need to solve for. So the net force equation is:

    Fnet = N - mg = ma

    solving for N:

    N = mg + ma

    now the problem im having is that a in the slowing down part is negative (its opposing motion) so while we cant solve for N directly, the equation states it should be less than 700 N (the original normal force before acceleration).

    my confusion comes from when i picture myself in an elevator moving up, and then slowing down to stop I feel that the scale/normal force should be larger (ie, I would feel heavier for those few seconds). Is that incorrect? Was my calculations correct?
     
  2. jcsd
  3. Sep 25, 2007 #2

    Doc Al

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    Staff: Mentor

    Yes, that is incorrect. (Pay attention next time you are on an elevator!) Maybe a different example will help. Imagine yourself sitting in a car riding along at constant velocity. If you slow the car, are you pushed harder into the seat back or pulled away from it? (The force of the seat back pushing against you is similar to the scale reading in the elevator.) What if you speed up?
    Yes.
     
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