A massless, frictionless pulley is suspended from a rigid rod

  1. A massless, frictionless pulley is suspended from a rigid rod. Two masses, m (50 kg) and M (60 kg), are suspended on either side of the pulley, respectively, by a light, inextendable cord. Determine the accelerations of the masses.

    I drew the force diagrams of both; there is an upward tension force and a downward gravity force in each. Mg - mg = (m + M)a. Now acceleration of block m is (M - m)g/(M + m) upward and block M's acceleration is the same but downward. So does m have a positive acceleration while M has a negative acceleration?

    Thanks.
     
  2. jcsd
  3. Integral

    Integral 7,349
    Staff Emeritus
    Science Advisor
    Gold Member

    Could be either, it depends upon how you define your coordinate system. Is postive up or down?
     
  4. I guess I define positive as going up on the coordinate sys. In the problem, m is pulled up while M goes down.
     
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