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Homework Help: Atwood machine with two ropes on one of the blocks

  1. Nov 14, 2013 #1
    1. The problem statement, all variables and given/known data
    Block 2 has a mass of 10kg and is attached to a pulley. The string going through the pulley is fixed to a bar on one end. The other end of the string goes over a pulley and a cylinder, of mass 20kg hangs on the string. The coefficient of friction between block 2 and the surface is 0.1 and the angle is 12 degrees.

    a) Draw force diagrams for the block and the cylinder.
    b) Write the net force equation for the cylinder.
    c) Write the net force equation for the block.
    d) What is the acceleration of the block and the cylinder?
    e) What is the tension in the rope?


    m1 = 20 kg
    m2 = 10 kg
    μk = 0.1
    θ = 12°

    2. Relevant equations
    Fnet = ΣF = ma
    N = mFgcosθ
    fk = μkN

    3. The attempt at a solution
    a) On block 1, tension pulls up and the force of gravity pulls straight down with a force greater than that of the tension. On block 2, there are two tension forces (one for the rope on each side of the pulley) pulling parallel to the incline upwards, friction in the opposite direction of the tension forces, the normal force pulling up and at an angle perpendicular to the plane's surface, and the force of gravity pulling straight down.
    b) Fnet cylinder = Fg - T
    = m1g - m1a
    c) Fnet block = T1 + T2 - Ffk - Fgsinθ
    = m2a/2 + m2a/2 - μFgcosθ - Fgsinθ
    = m2a/2 + m2a/2 - μm2gcosθ - m2gsinθ
    d) acylinder = ablock/2

    I would love to continue with this attempt at a solution, but I honestly do not know where to go from here... I am not even sure if my belief that the acceleration of block 1 is twice that of block 2. Basically, I am completely stuck, and I have no idea on how to go about solving for the acceleration of either block. Obviously, any help is very much appreciated. Thank you so much!
  2. jcsd
  3. Nov 14, 2013 #2


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    Science Advisor
    Homework Helper
    Gold Member

    The second line is wrong. Think about it again.
    Same error as before. You are equating ma with T in each case. What should you equate ma to?
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