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Weight in an elevator? Free body diagrams

  1. Mar 24, 2014 #1
    A 60kg student is standing on a scale in an elevator that measures weight in newtons. Calculate the reading on the scale if the elevator is accelerating down at 4 m/s^2.....when the elevator is accelerating up at 4 m/s^2.... and in free fall.

    Not sure if my work/answer is correct

    accelerating down (up is positive)
    Fnet= Fn - Fg
    ma = Fn - mg
    (60)(-4)= Fn - (60)(-9.8)
    -240 = Fn +588
    Fn = -828
    ∴ Fn = 828 N(Down)

    accelerating up (up is positive)
    Fnet = Fn - Fg
    240 = Fn +588
    Fn = -348
    ∴Fn = 348 N(down)

    Free fall: the weight of the student is the same as if he/she were standing on the ground(588N) since gravity is still present and the student's mass remains constant.
  2. jcsd
  3. Mar 24, 2014 #2
    You did it incorrectly, because the Fg should be added to the Fn, not subtracted. You take care of its direction by making its acceleration, g negative (so it points downwards). Also think about it intuitively- the student should press down on the scale "harder" if the elevator is accelerating upwards instead of downwards, but you got a lower value. In free fall, it's also incorrect. Imagine if you're falling and the elevator is falling as well. You feel "weightless", so what would the scale reading be?
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