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Homework Help: Four mass and Tension problem

  1. Jul 6, 2009 #1
    1. The problem statement, all variables and given/known data

    Four blocks are on a horizontal surface. The blocks are connected by thin strings with tensions Tx, Ty, Tz. The masses of the blocks are A=22.0 kg, B=21.0 kg, C=17.0 kg, D=27.0 kg. Two forces, F1=83.0N and F2=50.0N act on the masses as shown. Assume that the friction between the masses and the surface is negligible and calculate the tension Tx.

    See attached Pic.

    2. Relevant equations

    F = ma

    3. The attempt at a solution

    First, I calculated the acceleration of this system.
    a=(F1-F2)/(mA+mB+mC+mD). a = .3793m/s^2

    Then consider A and B as a whole.

    So F1-Tx=(mA+mB)*a

    Solve for Tx, which = 66.69N
    The correct answer is 74.7N... i really don't know what i'm doing wrong, any help / hint would be greatly appreciated.

    Attached Files:

  2. jcsd
  3. Jul 6, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Although I can't see your diagram, if I assume that the four blocks from left to right are:

    A B C D

    and that

    Tx is the tension in the rope connecting A and B
    Ty is the tension in the rope connecting B and C
    Tz is the tension in the rope connecting C and D

    and that

    F1 pulls to the left and acts on the leftmost end
    F2 pulls to the right and acts on the rightmost end

    then I get the right answer. I don't think your approach of considering both A and B makes much sense (because then you have three different forces to consider, two of which are unknown). You want to consider a block for which the only two forces acting are one that you already know, and Tx. In this case, that is block A. Set up a free body diagram just for block A and try it again.
  4. Jul 7, 2009 #3
    Yeah, your right. someone on my online class said to consider A and B as a whole but i guess thats wrong.
    thanks again!
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