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Homework Help: Another force problem

  1. Oct 3, 2007 #1

    lim

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

    A 57.0 kg girl weighs herself by standing on a scale in an elevator. What is the force exerted by the scale when the elevator is descending at a constant speed of 10 m/s?
    What is the force exerted by the scale if the elevator is accelerating downward with an acceleration of 2.0 m/s2?
    If the elevator's descending speed is measured at 10 m/s at a given point, but its speed is decreasing by 2.0 m/s2, what is the force exerted by the scale?

    2. Relevant equations
    F=ma, Fs-W=ma


    3. The attempt at a solution
    I got the first part, 9.8(57) = 558.6 N.

    For the second part I used Fs-W=ma, Fs - 558.6 = (57)2, Fs= 672 N. But that was wrong...?

    For the 3rd part, I used F=ma, but F= 57 (-2) = -144 N, and that wasn't right either.

    Help!
     
  2. jcsd
  3. Oct 3, 2007 #2

    Dick

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    You are making sign mistakes. For the second part write Fs+W=ma. Fs is always in the up direction, so lets call that the + direction. The weight acts down so I should put that in with a minus sign, ditto acceleration is down, so put that in with a minus sign. Fs-558.6N=(57kg)*(-2)m/s^2. Now what do you get? What happens in part three?
     
    Last edited: Oct 3, 2007
  4. Oct 3, 2007 #3

    lim

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    I'm confused about part three. They give me an acceleration(-2.0 m/s^2) and I have the mass (57 kg), but how do I tie in the velocity(10m/s)?
     
  5. Oct 3, 2007 #4

    Dick

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    The elevators downward speed is DECREASING. This means its acceleration is actually UPWARDS at 2m/sec^2. You do the same thing with the 10m/sec that you did in part one. The current speed doesn't have anything to do with the current acceleration.
     
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