What is the net force acting on a projectile at its highest point?

At the highest point of a projectile's trajectory, the only force acting on the projectile is the force of gravity. This occurs because, at that peak, the vertical component of the projectile's velocity is zero, meaning the projectile is momentarily at rest in the vertical direction before it begins to descend.

While it may seem that other forces, like air resistance, play a role, the correct understanding is that the net force is solely due to gravity acting downward. Air resistance does exist, but it does not negate the gravitational pull; rather, it opposes it. The force of gravity remains constant throughout the projectile's motion, and at the highest point, it is still the dominant force acting on the object.

Understanding that at the highest point the projectile is not in a state of equilibrium (where all forces cancel out) but is instead influenced solely by the gravitational force helps clarify why the assertion that only gravity is acting is accurate. Thus, it is reasonable to conclude that the net force in this condition is the gravitational force acting downward.