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Acceleration/Tension between two masses

  1. Jul 3, 2014 #1
    1. The problem statement, all variables and given/known data

    I am currently taking a Summer physics course, and today we were given a Force/Friction problem that even after the professor explained, I could not fully understand.

    https://de6d4c993829dd9a7ebe8c71bf32cb36b6fe5892.googledrive.com/host/0B79Pxd57IVb7aWU0Q3JqeG5XbjQ/FreeBodyFrictionPrblm.png [Broken]

    Mass of Objects: m1=5.0kg m2=2.0kg

    Given Friction Valules: Mk1 (Between Block and Surface) = 0.20 Mk
    Mk2 (Between Blocks) = 0.15

    I am supposed to find acceleration, as well as the tension between the two masses.

    2. Relevant equations

    Equations that he used to solve the problem:

    n-mg=0

    Force-Kinetic Friction-Tension = mass*acceleration

    3. The attempt at a solution

    I was able to draw a free body diagram of the system, however, I only included 2 values for normal force and Kinetic Friction, where in his complete Free Body Diagram, there were 3 values for both. I really got lost when he solved the problem using a system of equations towards the end.

    The acceleration ended up being 80/7 m/s2, or about 11.4 m/s2, and tension ended up being around 26 N.

    I have been studying this topic, but I don't really understand the fundamentals of the whole process, and memorizing formulas hasn't helped whatsoever. Any guidance would be greatly appreciated!
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 3, 2014 #2

    PhanthomJay

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    you should draw free body diagrams of each object separately. Identify all the forces acting on each object, and apply newtons laws. All accelerations are with respect to the ground. Note that the top and bottom objects have the same acceleration but in opposite directions. And the cord tensions on each are the same. No acceleration in vert direction. Do not draw a free body of the blocks together....split them up.
     
    Last edited by a moderator: May 6, 2017
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