At the very top of its trajectory, the net force on the rock is only the force of gravity.

Here's why:

* At the peak, the rock's vertical velocity is zero. It's momentarily stopped before it starts falling back down.

* Gravity is always acting on the rock. Even when the rock is moving upward, downward, or at rest at the peak, gravity is pulling it towards the Earth.

* There are no other significant forces acting on the rock. We're assuming no air resistance for simplicity.

Therefore, the net force is equal to the force of gravity, which is the rock's weight (mass * acceleration due to gravity).