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Homework Help: Using Newtons 2nd law for rotational motion

  1. Nov 18, 2012 #1
    I've attached an image explaining the problem. All pulleys are massless. Basically, they want me to show some relations between wire tensions, using newtons 2nd law for rotational motion.

    This law is written in my book as:

    dL / dt = torque, where L is angular momentum.

    I'm not sure exactly how to do that. Since there is no mass in the pulleys, they shouldn't have any angular momentum. And if they don't, how can I apply this law for anything? Or perhaps they just want me to state that since the pulleys are clearly falling straight down (no lateral movement), the string tensions on each end of each pulley has to be equal to each other?

    Any help here would be very appreciated!

    Attached Files:

    • oppg.png
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  2. jcsd
  3. Nov 18, 2012 #2


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    Although the pulleys themselves have no angular momentum, the forces in the strings have moment about the centres of the pulleys. If you write out the torque equations for the pulleys you should get the desired answers.
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