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Newton's second law and tension

  1. Sep 19, 2011 #1
    Newton's second law and tension!!!!

    1. The problem statement, all variables and given/known data

    A helicopter is lifting two crates simultaneously. One crate with a mass of 139 kg is attached to the helicopter by cable A. The second crate with a mass of 88 kg is hanging below the first crate and attached to the first crate by cable B. As the helicopter accelerates upward at a rate of 1.2 m/s2, what is the tension in each of the two cables (in kN)?

    2. Relevant equations

    Fnet=ma

    3. The attempt at a solution

    I drew a free body diagram and for the first crate (139 kg) I had tension pointing upwards and gravity pointing downwards. So I did the following calculations:

    -mg + Ta= ma
    Ta= m (a + g)
    = 139 (1.2 +9.8)
    = 1529 N
    = 1.529 kN


    For the second crate (88 kg), I had tension of the second crate pointing upwards, tension of the first crate pointing downwards and gravity pointing downwards. So I did the following calculations:

    -mg- 1529 + Tb= ma
    Tb= ma + mg + 1529
    = m (a+g) + 1529
    = 88 (1.2 + 9.8) + 1529
    = 2497 N
    = 2.497 kN

    Both answers were wrong so I'm not so sure what I'm supposed to be doing for this question. Any help will be appreciated!!!
     
  2. jcsd
  3. Sep 19, 2011 #2

    cepheid

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    Re: Newton's second law and tension!!!!

    Are you sure that those are all of the forces acting on crate A? After all, it has a rope (that is in tension) attached to the bottom of it as well...
     
  4. Sep 19, 2011 #3

    Dick

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    Re: Newton's second law and tension!!!!

    The first crate is the one with two tensions acting on it (Cable A and Cable B). The second crate is only attached to Cable B. Do the second crate first!
     
  5. Sep 19, 2011 #4
    Re: Newton's second law and tension!!!!

    So the calculations I did for crate A should actually be for crate B? and the calculations I did for crate B should be for crate A?
     
  6. Sep 19, 2011 #5

    Dick

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    Re: Newton's second law and tension!!!!

    Sort of. Your logic was swapped. But don't just reverse the numbers. They will change when you redo it.
     
  7. Sep 19, 2011 #6
    Re: Newton's second law and tension!!!!

    If I'm not only supposed to reverse the numbers then how exactly am I supposed to solve this problem??
     
  8. Sep 19, 2011 #7

    cepheid

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    Re: Newton's second law and tension!!!!

    Exactly as you were intending to solve it. Draw a free body diagram for crate B. Solve for the forces on it. Draw a free body diagram for crate A. Do the same.

    What, specifically, is the source of your confusion?
     
  9. Sep 19, 2011 #8
    Re: Newton's second law and tension!!!!

    I understand how to go about doing the question now. thank you for all of your help :)
     
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