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Homework Help: Acceleration in an elevator and Physical Laws

  1. Mar 13, 2010 #1
    1. The problem statement, all variables and given/known data
    I've gone out and measured my changing weight in an elevator using a bathroom scale, taking measurements each second, with the elevator ascending 4 storeys. Im wondering if it is possible to work out the acceleration of the elevator from these results, and if so how they relate to Newton's 2st and 3rd law.
    e.g.1 mass = 75kg
    mass inside elevator = 81kg
    e.g.2 mass = 75kg
    mass inside elevator = 70kg
    2. Relevant equations
    F=ma
    g=9.8
    Those should break down to give these?
    F = m * (g + a)
    a = F / m – g
    3. The attempt at a solution
    e.g.1 a = (81 * 9.8)N / 75kg – 9.8ms-2
    a = 0.784ms-2 Upwards

    e.g.2 a = (70 * 9.8)N / 75kg – 9.8ms-2
    a = -0.653ms-2 Upwards
     
  2. jcsd
  3. Mar 13, 2010 #2

    rl.bhat

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    HI Tomdoml, welcome to PF.
    When the elevator is not moving the weight is
    F1 = m*g. = 75 kg.wt.
    When the lift is moving up, the weight is
    F2 = m(g+a) = 81 kg.wt.
    Now take the ratio and find the acceleration.
     
  4. Mar 13, 2010 #3
    So would my attempted solutions be correct in determining the acceleration in the elevator?
     
  5. Mar 13, 2010 #4

    rl.bhat

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    Correct.
     
  6. Mar 13, 2010 #5
    The one things confusing me in regards to this experiment is the effect that newtons 3rd law has on the results. Since the elevator pushes me up, i push back down with equal force which is how i figure out the acceleration of the elevator, but if the forces were equal wouldnt that just result in the elevator not moving at all?
     
    Last edited: Mar 13, 2010
  7. Mar 13, 2010 #6

    rl.bhat

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    The action and reactions equal and opposite, but they act on different objects
     
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