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Homework Help: Free body diagram

  1. Mar 14, 2014 #1
    1. The problem statement, all variables and given/known data

    Can someone explain why the FBD of Block C would involve three cord tensions?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 14, 2014 #2

    Doc Al

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    Staff: Mentor

    Whoever is claiming that is probably considering the bottom pulley, which is connected to block C, as part of the block C system. Perfectly OK.

    But you certainly do not have to! If you consider block C separately, you'll also need a diagram for that bottom pulley.
  4. Mar 14, 2014 #3


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    Staff Emeritus
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    Homework Helper

    It can't. There are only two cords directly connected to block C.

    Now, if you were to make a horizontal cut between the two smaller sheaves, you would have the tension in three cords to consider.
  5. Mar 14, 2014 #4


    so here they've considered the block + the bottom pulley as the system?
    is there any reason for why they did that instead of just considering Block C as the system?
  6. Mar 14, 2014 #5

    Doc Al

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    Staff: Mentor

    Yes, for the purpose of that FBD they consider 'block + bottom pulley' as a single unit.

    Just saves a bit of time. If you just considered Block C alone, you'd have to do another FBD for the bottom pulley to figure out the tension in the cable connecting it to the block. No big deal; either way is fine.
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