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Homework Help: Tension on the String

  1. Apr 11, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    A FBD on the bottom block gives tension in C is equal to Mg. Then, a FBD on the middle block should give T_b-T_c=0, so tension on B should also be equal to Mg. Finally, a FBD on the top block gives the tension on A is equal to Mg.

    But this is not correct, what did I do wrong?
  2. jcsd
  3. Apr 11, 2015 #2


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    Gold Member

    Really? You might want to rethink that.
  4. Apr 11, 2015 #3
    What are the forces acting on the middle block? Are they not only tension in B and tension in C?
  5. Apr 11, 2015 #4
    Draw a free body diagram of the middle block. Show the forces acting on that block. What are they?

  6. Apr 11, 2015 #5


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    For your 2nd FBD, on the middle block, you forgot to include the Weight of that block. Same for the FBD on the top block.
  7. Apr 11, 2015 #6


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    Gold Member

    Yes, but how does that relate to the answer to the you are looking for. Are you looking for the forces on a single block or the tension on the various strings? Suppose B was a rigid rod instead of a string. What would that do to your perception of the tension in string A ?
  8. Apr 11, 2015 #7
    Ok, thanks. I got it now. The FBD on B should be T_b=T_c+Mg
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