Final answer:
The net force that accelerates the wagon and horse system is calculated as the difference between the pull force and the frictional force, which is P minus f (P - f).
Step-by-step explanation:
The net force that accelerates a system, in this case, the wagon and the horse, is found by taking into account all the external forces acting on the system and adding or subtracting them as vectors based on the direction they act. The force P represents the pull between the wagon and the horse, which is an internal force and therefore does not contribute directly to the net external force. The force f is the frictional force acting against the direction of motion, and F is the reaction force from the ground that propels the horse and wagon forward. Following Newton's second law, and considering the direction of each force, the net force that accelerates the system is P minus f (P - f), as F and P are essentially action-reaction pairs according to Newton's third law and don't contribute to the net external force that actually accelerates the system.