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Homework Help: Modified Atwood Machine Problem

  1. May 14, 2012 #1
    1. The problem statement, all variables and given/known data
    Consider a modified Atwood machine, where one mass of 20 kg lies on a flat tabletop, and another mass of 5 kg hangs off the edge of the table, where the two masses are connected to each other by a massless string and a frictionless pulley on the edge of the table. The coefficient of kinetic friction between the tabletop and the 20 kg mass is .100. Find a) the acceleration of the system, and b) the tension in the string connecting the masses.

    2. Relevant equations
    Fnet = ma
    f = (.1)N, where f is the force of kinetic friction and N is the normal force
    g = -10 m/s/s (for easy calculations)

    3. The attempt at a solution
    a) The acceleration of the object (or system) is the net force divided by the total mass of the system. The calculation for net force I executed: (5 kg)*(10 m/s/s) - (.1)(200 N) = 50 - 20 = 30 N. 30 N/ 25 kg = 1.2 m/s/s = a.

    b) I'm more unsure about this part. The net force on the 5 kg block is (5 kg)*(10 m/s/s) - T = (5 kg) * a. a must be the same for both masses, so 5 kg * 1.2 m/s/s = 6 N. 6 = 50 - T. - T = -44. T = 44 N.

    The part I'm unsure about is whether the fact that the tabletop and the 20 kg block having friction associated with them should factor into the calculation of the tension.

    I'd like to know if my two answers are correct. If they are, why is it that part b doesn't have the friction force factored in. If something is wrong, please explain the correct answer.

  2. jcsd
  3. May 14, 2012 #2
    After some fiddling, I figured out why my answers are correct.
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