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Homework Help: Mass of stone

  1. Nov 28, 2012 #1
    A stone is suspended from the free end of a wire that is wrapped around the outer rim of a pulley, as shown in the figure. The pulley is a uniform disk with mass 11.3kg and radius 49.0cm and turns on frictionless bearings. You measure that the stone travels a distance 12.3m during a time interval of 2.50s starting from rest.

    1. Find the mass of the stone.

    2. Find the tension in the wire.

    Free fall acceleration = 9.80 m/s^2

    Figure: http://tinyurl.com/cpht4rr

    I'm not to sure what formula to use for this?
  2. jcsd
  3. Nov 28, 2012 #2
    EDIT: Working on it atm, I'll see what I get first.
  4. Nov 28, 2012 #3


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    Is the stone or the pulley in free fall?

    Hint: draw the free body diagram first.

  5. Nov 28, 2012 #4
    It's for both of them atm i'm using
    then I will apply newtons second law mg - T = ma
    hopefully this will get me the mass.
  6. Nov 28, 2012 #5
    Ok so I got S = 0 +1/2 (9.8m/s^2)(2.5s)^2 which gave me 30.625
    30.625/ 12.3 = 2.489...m/s^2

    I manipulated newtons second law to get T = 1/2M*a
    T = 1/2(11.3kg)(2.48...m/s^2)
    T = 14.067N

    mg-ma = T = m(g-a)
    m= T/(g-a)
    14N/(9.8m/s^2-2.4...m/s^2) = 1.92kg

    How does this look?

    Mhm got it wrong, not sure where the mistake is though.
  7. Nov 28, 2012 #6


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    The stone accelerates with a and s= 1/2 at2, why do you calculate with g?

    What about the pulley? Does it move?

    Last edited: Nov 28, 2012
  8. Nov 28, 2012 #7
    Yes it says "take free fall acceleration to be 9.80 m/s^2"
  9. Nov 28, 2012 #8


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    It is the free-fall acceleration, but is not the acceleration of the stone. Calculate the acceleration again.

  10. Nov 28, 2012 #9
    Dope! Haha thanks for that, I got 3.936 m/s^2 meaning the tension is 22.2384 N.
    Therefore the mass comes out to be...3.79 kg.

    Thanks a lot! :P
  11. Nov 28, 2012 #10


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    That is better:smile:

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