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Deriving atwood's gravity formual

  1. Aug 28, 2003 #1
    I have always been curios to how Atwood derived his formula a=g(m2+m1)/(m2-m1) to get gravity. Does anyone know how he did it? I have read that he used Netwons f=ma but I personally could not find a way to do this using newtons f=ma.

  2. jcsd
  3. Aug 28, 2003 #2


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    The formula is:

    g = a(M1 + M2)/(M1- M2)

    using F = ma and F = mg

    Gravity is acting on both the weights therefore:

    F1 = M1g

    F2 = M2g

    The total force acting down is given by:

    F = F1 - F2

    which is:

    F = g(M1 - M2)

    As the force is pulling both weights we can view the two weights as one single weight being acted on with a mass of M1 + M2, so using F = ma

    F = a(M1 + M2)

    We,ve now got two terms for the same force so we can subsitue in for F:

    a(M1 + M2) = g(M1 - M2)

    changing this around:

    g = a(M1 + M2)/(M1- M2)

    Last edited: Aug 28, 2003
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