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Homework Help: Three Blocks: missing mass

  1. Nov 2, 2008 #1
    1. The problem statement, all variables and given/known data

    Three blocks are pulled along a frictionless surface. The masses of two of the blocks and the tensions in two of the cords are given. Find the mass of the third block (Block 2).

    Block 1 ------- Block 2------Block 3 ------>
    Block 1: m1 = 12 kg
    Block 2: m2 = UNKNOWN
    Block 3: m3 = 35 kg
    Tension between Block 2 and Block 3: 111 N
    2. Relevant equations

    Newton's 2nd Law: F = ma

    3. The attempt at a solution

    111N = (m1+m2)a = m3a <-- is this true?

    Block 2, m2 = 35-12? This seems to easy to be true.
  2. jcsd
  3. Nov 2, 2008 #2
    That does not seem correct. You stated that the tensions in two cords are known, but only give the tension between Block 2 and 3. What is the other known tension?

    Once you have that, draw free body diagrams, it might help to clear things up.
  4. Nov 2, 2008 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You said two cord tensions are given, but seem to have left one of them out here?

    111N = (m1+m2)a is correct.
    But it is not equal to m3a ... there are two forces contributing to m3's motion: the 111N and ____ ?
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