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Homework Help: Finding acceleration of an Atwood's machine

  1. May 8, 2014 #1
    1. The problem statement, all variables and given/known data
    Two masses are suspended from a pulley:


    mass of pulley = 0.20kg
    radius = 0.15m
    constant torque = 0.35Nm due to friction between the rotating pulley and its axle
    m1 = 0.40kg
    m2 = 0.80kg

    What is the acceleration of the suspended masses?

    2. Relevant equations
    Fnet = I[itex]\alpha[/itex]

    3. The attempt at a solution
    I tried to make sense out of this problem using the equation from above initially, but my acceleration was way too big. I know the acceleration is supposed to be somewhat big anyway but it was too big. So instead I searched for helm on google first and found an equation which told me to do this:

    I = (1/2)mr2 = (1/2)(0.20)(0.5)2 - 0.025kgm2

    a = [g(m1 - m2) - [itex]\tau[/itex]/r]/(m1 + m2 + I/r2)
    a = 9.8(0.80+0.40)-(0.35/0.15)/(0.80+0.40+(0.0025/(0.15)2)
    a = 1.23 m/s2

    If this is right, how can I clean it up to make it relevant to how Fnet = I[itex]\alpha[/itex] is supposed to be set up?
  2. jcsd
  3. May 8, 2014 #2
    Check your formula:
    Fnet I[itex]\alpha[/itex]
  4. May 8, 2014 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You can't arbitrarily pick an equation out of a book and use it without understanding it, even in the unlikely event that it happened to be the right equation. Instead, use free body diagrams of each block and the pulley, and use newton's laws for translational or rotational motion on each piece.
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