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Elevator Problem

  1. Sep 26, 2014 #1
    I don't understand how the forces work in this problem. Gravity points down and I make it be negative direction. Then an equal but opposite force points up. Then there is another acceleration going down since the elevator is slowing down. This make the total acceleration downwards even larger. So wouldn't the person weigh more, not less?
     
  2. jcsd
  3. Sep 26, 2014 #2

    A.T.

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    The weight of the person is always the same and pointing downwards. The scale shows the upwards contact force on the person. If the person accelerates downwards, the net force must be downwards, so the upwards force (scale) must be less than the downwards force (weight).
     
  4. Sep 27, 2014 #3

    TumblingDice

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    Bathroom scales are calibrated for the acceleration factor of Earth's gravity (32 ft/sec^2). When an elevator begins upwards acceleration, this acceleration is added to the acceleration component of Earth's gravity. That results in more force downwards from your inertial mass on the bathroom scale, and the scale indicates a greater value.

    If the elevator reaches a steady velocity upward, gravity is now the only acceleration force and the bathroom scale will indicate the normal value for your weight.

    Once the elevator begins 'braking', an acceleration component opposite to that of gravity is applied, which now subtracts from the acceleration force of gravity, reducing the net acceleration, and the bathroom scale indicates a lesser value than normal.
     
  5. Sep 28, 2014 #4

    CWatters

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    but is that acting on the person or the elevator? The scales are "squeezed" between the two.
     
  6. Sep 28, 2014 #5
    of course, you should choose B.
     
  7. Sep 29, 2014 #6
    Logan3,

    Have you drawn a free body diagram of the person? What are the external forces acting on the person? Write down your equation for the force balance using Newton's second law.
     
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