Final answer:
At the highest point in its trajectory, a rock that has been thrown straight up experiences gravity as the net external force acting upon it, which is the product of its mass and the acceleration due to gravity (mg).
Step-by-step explanation:
The question asks, "What is the net external force acting on the rock when it is at the top of its trajectory?" To answer this, we must understand the forces acting on an object in motion. At the highest point in its trajectory, an object like a rock thrown straight up has zero kinetic energy because its velocity is zero for an instant before it starts to fall back down.
This point is where the rock has maximum potential energy. The only net external force acting on the rock at this point is the force of gravity, which is always directed towards the center of the Earth. In the absence of air resistance, this force of gravity would be the only force acting on the rock, and the rock would be in free-fall.
In the real world, air resistance acts upwards, opposing the motion. However, when the rock reaches the highest point in its trajectory, its velocity is zero, so the air resistance is momentarily negligible, and gravity remains the only significant force. Thus, the net external force on the rock at the top of its trajectory is equal to its weight, which can be calculated as mg (mass times the acceleration due to gravity).