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Homework Help: What is the tension in the string?

  1. Feb 21, 2008 #1
    1. The problem statement, all variables and given/known data
    3.7 kg block A and 6.0 kg block are connected by a string of negligable mass. Force FA = (13 N) acts on block A; force FB = (24 N) acts on block B. What is the tension in the string? (both force FA and FB are to the right)

    2. Relevant equations

    fnet = ma

    3. The attempt at a solution

    Block A FBD:
    Weight force down of m*g = 36.297N
    Normal force up (opposite of weight)
    frictional force to the left = f1
    FA to the right = 13N
    Tension, T, to the right

    Block B FBD:
    Weight force down = 58.8N
    Normal force up (opposite of weight)
    frictional force to the right = f2
    FB to the right = 24 N
    Tension force, T, to the left

    Fnet Ax = 3.7kg * a = 13N + T - f1
    Fnet Bx = 6kg * a = 24N -T + f2
    Fnet of A and B together = 9.7 kg * a = 37N + f2 -f1

    Problem: four unknowns and only three equations...
  2. jcsd
  3. Feb 21, 2008 #2


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    Science Advisor
    Homework Helper
    Gold Member

    One of those equations is extraneous, so you really just have 2 equations with 4 unknowns. If the surface is frictionless, f1 and f2 are 0; if friction is involved, the problem needs to state the friction coefficients, otherwise, you can't get a numeric answer. IF friction is involved, you made an error on the sign of f2..why did you have it as plus? (I'm guessing that the floor is horizontal and frictionless? If friction were presnt, the system might not move).
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