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Homework Help: How do I calculate mass of pulley and force of friction on AtwoodMach.

  1. Feb 16, 2014 #1
    1. The problem statement, all variables and given/known data
    Basically, I need to calculate the mass of the pulley and friction using data that I collected.

    2. Relevant equations
    I was given: (m1 - m2)g = (m1 + m2 + mp)a + f

    3. The attempt at a solution

    For the first part, I collected data using a sensor to find the acceleration. I kept the total mass attached to each cup on each side of the machine constant but changed the difference in their masses by 10g each run.

    After 6 runs, I plotted my data of Fnet vs Acceleration. Fnet is (m1 - m2)g (I converted g into dynes). When I get my slope, the lab manual says that i should "expect our plot of (m1 - m2)g vs a to have a slope larger than (m1 - m2)g by an amount equal to the effective mass of the pulley mp and a positive intercept equal to the frictional force f"

    My equation turned out to be y = 143.28x + 323.24 (i used best fit line).

    This means 143.28 = m1 + m2 + mp
    In which case mp= 28.5g right?

    Then the force of friction should = 323.4 right?
    Is the unit in dynes?

    For part b, did the same procedure but kept the net force constant instead of the total mass. I gathered my data and plotted

    1/a vs m1 + m2

    I got y = 0.0002x + 0.0059 as my best fit line

    Now I have to use this equation to find the force of friction

    1/a = ((m1 + m2) )/((m1-m2)g-f) + mp/((m1-m2)g-f)

    But i get 4950 which I believe is in Netwons and that would be too much considering 323 dynes = 0.00323 newtons

    Attached Files:

  2. jcsd
  3. Feb 16, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You seem to be working the equations correctly - they may be the wrong equations of course, so you should check that you understand them. Don't just assume they are right: check!

    The first thing you need to do is check those units - do that by dimensional analysis.
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