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A massless, frictionless pulley is suspended from a rigid rod

by Soaring Crane
Tags: frictionless, massless, pulley, pulleys, rigid, suspended
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Soaring Crane
Dec31-03, 12:24 PM
P: 484
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?

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Dec31-03, 12:26 PM
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PF Gold
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P: 7,337
Could be either, it depends upon how you define your coordinate system. Is postive up or down?
Soaring Crane
Dec31-03, 12:35 PM
P: 484
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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