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Homework Help: Why does tension = -RT

  1. Nov 25, 2014 #1
    1. The problem statement, all variables and given/known data
    This figure shows a uniform disk, with mass M = 2.5 kg
    and radius R = 20 cm, mounted on a fixed horizontal axle.
    A block with mass m = 1.2 kg hangs from a massless cord
    that is wrapped around the rim of the disk.

    Sample problem 10-8 (has picture diagram and detailed solution): http://astro1.panet.utoledo.edu/~mheben/PHYS_2130/Chapter11-1_mh.pdf

    2. Relevant equations
    T – mg = ma
    torque = I α
    -RT ??? = (1/2)MR^2 α
    a = a(subscript t) = α R

    3. The attempt at a solution
    I understand how to do the problem but what I don't understand is how they got -RT. Torque is FrSin(theta) and the sin of 90 degrees is 1 not -1 so I would really appreciate any insight on how they got -RT.
  2. jcsd
  3. Nov 25, 2014 #2
    Well, it looks like positive direction is chosen to be up and vector T is pointing down so...
  4. Nov 25, 2014 #3
    Ok thanks
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