Final answer:
To determine the magnitude of the net force on the ball at the top of its trajectory, consider the forces acting on it. At the top of the ball's trajectory, the net force acting on the ball is zero.
Step-by-step explanation:
At the top of the ball's trajectory, the net force acting on the ball is equal to the force of gravity acting on it. Since the ball is at rest momentarily at the top, the net force is zero, as the gravitational force pulling the ball down is balanced by the force exerted by the ball in the opposite direction.
Since the ball is momentarily at rest at the top, the net force would be equal to zero at that instant. The gravitational force is balanced by the force of air resistance. Hence, the magnitude of the net force on the ball at the top is zero. This means there is no acceleration acting on the ball at that point.