Chapter 2 - Section 2.4 - Inertia and Equilibrium: Newton's First Law of Motion - Checkpoint - Page 37: 2.4

The sum of the forces due to the three cords is not the net force on the pulley. There is also a force pulling the pulley toward the foot, and the force of gravity pulling downward on the pulley. The sum of all these forces is the net force on the pulley.

The sum of the forces due to the three cords is not the net force on the pulley. To find the net force, we need to include all the forces that are exerted on the pulley. In addition to the three forces exerted by the three cords, there is also a force pulling the pulley toward the foot, and the force of gravity pulling downward on the pulley. The sum of all these forces is the net force on the pulley.