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Net torque question? Block 1 and block 2 hanging on pulley?

  1. Nov 12, 2012 #1
    In Fig. 10.45, block 1 has mass m1=460g, block 2 has mass m2=500g, and the
    pulley, which is mounted on a horizontal axle with negligible friction ahs radius
    R=5.0cm. When released from rest, block 2 falls 75.0cm in 5.00 s without the
    cord slipping on the pulley.

    (e) What is its rotational inertia?

    problem and picture on page 2 problem #55

    http://faculty.kfupm.edu.sa/PHYS/marzoug/Phys101/chapter 10 problmes.pdf




    Ʃτ = (T2 - T1) R

    Why doesn't mg1 or mg2 get calculated into the net torque and only the tensions?



    thanks!
     
  2. jcsd
  3. Nov 12, 2012 #2

    Simon Bridge

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    That happens later - there are three free-body diagrams.
    It's good discipline for more complicated problems: the pulley knows nothing about the masses - all it feels is these strings pulling on it.
    Note - the 1 and the 2 belong on the "m"'s not the "g"'s ... g is a constant.
     
  4. Nov 12, 2012 #3
    thanks!
     
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