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Homework Help: Incline with Pulley, find the mass of one block.

  1. Jun 19, 2013 #1
    1. The problem statement, all variables and given/known data

    A 20.0 kg block rests on a frictionless inclined plane of slope angle 30.0 degrees. A light cord attached to the block passes over a frictionlesss pulley at the top of the plane and is attached to a second block. What must be mass of the second block if the system is to be accelerating up and to the right at 2.00 m/s2?


    Is a link to the image.

    I found in class that the answer is 17.7 kg.. however I am stumped on where the numbers should go.

    2. Relevant equations

    3. The attempt at a solution
    http://hyperphysics.phy-astr.gsu.edu/hbase/incpl.html#c1 is the link where I use my formulas

    Well I do know that if I isolate the mass on the incline

    ƩFnetx = -m1g * sin(30°) + Fτ = 2*20

    I got 2*20 because of ma, and I know for m1g I substitute (20*9.8) I also know that acceleration is going up the incline, and if I were to remove the pulley and put them on the x-axis, it would be going to the right which is positive, therefore, the acceleration is positive.

    Solving for that I get, Fτ = 138N which I know to be true

    Now.. isolating m2 I get
    ƩFnetx = -Fτ + m2g = 2m

    however I have two masses??

    In a previous problem when I had to find the mass of m2 if I wanted the objects to be at rest or constant velocity (so I assumed F=ma, but the sum would be 0). I ended up finding the weight of M2 being 98N.. would I plug that in for m2g? I guess not considering it doesn't give me the answer.. but I just wanted some feedback on this.
  2. jcsd
  3. Jun 19, 2013 #2


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    Your last equation should read ƩFnetx = -Fτ + (m_2)g = 2(m_2). Now do the algebra correctly to solve for m_2!
  4. Jun 20, 2013 #3
    So I write the Equation..

    m_2 = -138 + m_2g

    However, I still scratch my head at this, and I'm not sure if substituting would work either. Sorry if my algebra skills seem to be lacking... it is 1:37 a.m. where I am at currently.
  5. Jun 20, 2013 #4
    Bring all of the mass dependant quantities to one side and factor it out. Divide the -Ft by accel - grav. You should end up with m_2 = (-138.1)/(2-9.81)
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