Boxes and a Pulley

  1. In the figure below, three ballot boxes are connected by cords, one of which wraps over a pulley having negligible friction on its axle and negligible mass. The masses are mA = 22.0 kg, mB = 40.0 kg, mC = 30.0 kg.



    Now I apply Fnet = ma so. . .

    the massA in the x direction would be T1 (tension) = (mA)(ax) and in the Y direction n-(mA)(g) = (mA)(ay).
    the massB in the y direction would be T1-T2 - (mB)(g) = (mB)(ay)
    the massC in the y direction would be T2 - (mC)(g) = (mC)(ay)

    but in the equations above wouldnt acceleration in the x direction (ax) = 0? and accel. in the y (ay)direction be (9.8 m/s^2 aka gravity (g))
  2. jcsd
  3. learningphysics

    learningphysics 4,120
    Homework Helper

    No ax isn't 0. The acceleration of mA towards the left equals the acceleration of the hanging masses downwards...

    using your conventions ax = -ay.

    so you have 3 equations with 3 unknowns... T1, T2 and ax.

    One "trick" you can use to simplify calculating T1 and ax, is to take mB and mC together as one system. that way you get 2 equations with 2 unknowns, T1 and ax.

    But the 3 equations you have seem fine.
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