1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    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:


    Attached Files:

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

    Doc Al

    User Avatar

    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 :).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook