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Moment of inertia and Atwood Machine

  1. Dec 17, 2012 #1
    1. The problem statement, all variables and given/known data

    The question is from a lab. The lab used an Atwood Machine with two masses suspended string looped over a pulley. The time for one mass to reach the ground was measured and acceleration of the mass was calculated. The results were plotted on a graph of weight difference (Y) vs acceleration (x).

    The question is from the linear fit, slope value, calculate the moment of inertia of the pulley.

    Linear Fit equation y=mx+b

    m(slope): 6.762 g/m/s^2

    2. Relevant equations

    I= 1/2mr^2

    3. The attempt at a solution

    Many attempts at trying to determine the moment of interia from the slope, however I get lost because the radius of the pulley was not measured in the lab. The assumption is made that pulley it self in frictionless.

    Thanks in advance
  2. jcsd
  3. Dec 17, 2012 #2


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    Hello doub. Welcome to PF !

    Do you know the equation of motion for Atwood Machine ?

    You need the two mass values as well as the radius of the pulley. The radius of the pulley gives the connection between the acceleration of the two masses and the angular acceleration of the pulley.

    See the Wikipedia article for an Atwood Machine: http://en.wikipedia.org/wiki/Atwood%27s_machine
    Last edited: Dec 17, 2012
  4. Dec 17, 2012 #3
    I know the two mass values, however we do not know the radius of the pulley.

    In the first trial the m1 mass is 122.59 g and the m2 mass is 113.46 g with the accelerations calculated as 0.07 m/s^2

    Trial 2 m1 mass 124.34 g m2 mass 111.71 g with the acceleration calculated as 0.21 m/s^2

    And a correction on the slope. The slope value s 5.801 g/m/s^2

    I'm not sure if that clarifies things
  5. Dec 17, 2012 #4


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    The best that you can do is to fine I/r2 .

    You'll have two values for that. One from each trial.
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