Consider the diagram below
All circles are pulleys, which are fixed at the ceiling. M represents an engine (with a certain power) that is pulling the cable. C and D are weights. The acceleration of the system is 0 (D is moving with constant velocity). Assume the pulley and the rope have no mass, and friction is negligible.
One can calculate the tensions on the pulley from which C hangs by diving C's weight by 2. However, the tension also depends on the mass of D, right? The previously mentioned calculation doesn't take this into account.
My question is, if the mass of D varies, what happens to the tension on the left cable? Does it remain constant and it is the engine that has to compensate for the variation of mass? Or does the tension vary?