Acceleration of falling balls
The nature of the rubber bands is not explained and I don't think relevant; I gave all the information given in the problem. The 3M ball (on top) has the downward force of gravity and the downward tension from the M ball (below it) and the upward tension from the top rubber band (the tension being from the masses of both of the balls, so 4M) initially, correct? So when you remove the M ball on the bottom, I believe the 3M ball has just the downward force of gravity from its own mass and the upward 4M tension from the rubber band. (Please correct if I'm wrong about this). So the M ball will fall normally, but the 3M ball will accelerate upwards due to the 4M tension. But how fast is that?