A massless, frictionless pulley is suspended from a rigid rod

[Soaring Crane]

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.

 

[Integral]

Could be either, it depends upon how you define your coordinate system. Is positive up or down?

 

[Soaring Crane]

I guess I define positive as going up on the coordinate sys. In the problem, m is pulled up while M goes down.