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I Torque positive vs. negative

  1. Dec 4, 2016 #1
    I'm having trouble understanding this attached example in my physics textbook. It notes torque as being positive when substituted back into mg-Ft=-ma rather than being negative which I would think it would be because it is going in the clockwise direction. Can anyone help me understand why this is?

    Ex: String is wrapped around a uniform solid cylinder (like a yo-yo) of mass M and radius R, and the cylinder starts to fall from rest, find the acceleration:
    They do Ftension-mg=-ma (which I understand)
    And then look for torque: torque=Ftension*R=I*angular acceleration. <-- This is where I diverge from what they do since I did torque=Ftension*R= - I * angular acceleration
    They end up with ⅔ g, and I get 2g. Their answer obviously makes more sense so I don't understand why torque would be positive.
    Also by the way, I is 1/2MR^2

    Thank you for any help!
  2. jcsd
  3. Dec 4, 2016 #2

    Stephen Tashi

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    Science Advisor

    Torque and angular acceleration are each vectors. In 2D problems, the "+" or "-" value indicates whether a vector points "into the page" or "out of the page". In the vector statement of the equation "torque = (moment of intertia)(angular acceleration), both the torque vector and the angular acceleration vector point in the same direction.
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