Two boxes are suspended from a rope over a pulley. each box has weight 50 n. what is the tension in the rope

We can draw a free body diagram for one of the boxes. There are two forces acting on the box. The downward force is the force of gravity which is 50 N. The upward force is the force from the tension in the rope. Since the box is suspended, it is not accelerating. Therefore, the net force must be zero. Therefore the tension in the rope must be 50 N.

This is what a calorimeter is used for simpler ex, to measure calories in food.