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Can someone explain this?

  1. Dec 20, 2006 #1
    1. The problem statement, all variables and given/known data
    A 15.0-kg monkey hangs from a cord suspended from the ceiling of an elevator. The cord can withstand a tension of 200 N and breaks as the elevator accelerates. What was the elevator's minimum acceleration (magnitude and direction)?

    2. Relevant equations

    3. The attempt at a solution
    I labeled the force of rope as Fr, so:

    Fr - mg = ma
    (200 N) - (15.0 kg)(9.8 m/s^2) = (15.0 kg)a
    a = 3.53 m/s^2

    This is the answer I came to. My question is why is 200 N plugged into the equation above? I don't see any other way to do it, but the problem states that the cord can withstand a tension of 200 N, so if I plug in 200 N the cord still wouldn't break? But then again I don't see how else to do it.
  2. jcsd
  3. Dec 20, 2006 #2


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    Staff Emeritus
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    Gold Member

    Another way of looking at this question is find the maximum acceleration of the elevator if the maximum tension the cord can withstand is 200N. If the elevator accelerated at a greater rate the cord would break. However, we know that the cord did break, therefore we can say that for the chord to break the inequality a>3.53 must be true. You however, are quite correct, if the elevator accelerated at exactly 3.53m/s2, then the cord would not break. If the acceleration was only slightly greater than this value then it would break. Therefore, this value of acceleration is said to be the minimum value at which the cord will break.

    I hope that makes sense.
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