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

1. M (mass on left): There are two forces acting on M: tension force (upward) and weight force (downward). Since the mass is accelerating downwards, the net force must be downwards. Therefore, weight > tension. We represent the weight force by an arrow pointing downwards, and tension force by an arrow pointing upwards. 
2. m (mass on right): There are two forces acting on m: tension force (upward) and weight force (downward). Since the mass is accelerating upwards, the net force must be upwards. Therefore, tension > weight. We represent the weight force by an arrow pointing downwards, and tension force by an arrow pointing upwards.

Option A is incorrect because it shows the tension force acting downwards on m, which is incorrect as the mass is moving upwards. 
Option B is incorrect because it shows the weight force acting upwards on m, which is incorrect as the mass is lighter and should accelerate upwards. 
Option D is incorrect because it shows the tension force acting downwards on m, which is incorrect as the mass is moving upwards.

Two Unequal Masses M and M (M > M) Are