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Simple Atwood apparatus question

  1. Jul 2, 2009 #1
    I had this question on the MCAT earlier, and I'm not sure if I did it properly.
    There is an Atwood machine such that a person of mass m is holding onto both ends of the string. What is the tension in the string? According to my calculations, since a=0 since the string is not moving, T=0.5m*g. I think if you would cut the person in half, each half, m1 and m2, would have a mass of 0.5m. Thus:
    T-m1g= ma
    m2g-T=ma
    So a=0 since there is no acceleration and m1=m2=0.5m. This gives us T=0.5mg


    Did I approach this correctly? I've enclosed a picture of the problem:

    http://img217.imageshack.us/i/68744608.jpg/
     

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    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jul 2, 2009 #2

    Doc Al

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    Staff: Mentor

    Your answer is correct. Rather than cut the person in half, just realize that the ropes pull up on the person twice:
    2T - mg = 0, thus T = mg/2.
     
  4. Jul 2, 2009 #3
    Thanks for your help! That makes more sense.
     
  5. Jul 2, 2009 #4
    Yeah, pulleys can be counter-intuitive if you are not careful. So I tend to be extra careful when dealing with them :).
     
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