Two unequal masses M and m (M m)are connected by a light cord passing over a pulley of negligible mass,as shown in the figure.When released,the system accelerates.Friction is negligible.Which figure below gives the correct free-body force diagrams for the two masses in the moving system?

Question

Two unequal masses M and m (M > m)are connected by a light cord passing over a pulley of negligible mass,as shown in the figure.When released,the system accelerates.Friction is negligible.Which figure below gives the correct free-body force diagrams for the two masses in the moving system?

Quizplus · Accepted Answer

For mass M, the weight W is acting downwards and tension T is upwards in the string. Since the system is accelerating downwards, the net force acting on mass M must be downwards. This can be achieved by subtracting the tension force from weight force which creates a net force of (W - T) acting downwards.
For mass m, the weight W is acting downwards and tension T is upwards in the string. Since the system is accelerating downwards, the net force acting on mass m must be upwards. This can be achieved by subtracting the weight force from tension force which creates a net force of (T - W) acting upwards. 
Therefore, the correct free-body force diagrams for the two masses in the moving system is shown in option C.

Two Unequal Masses M and M (M > M)are Connected