Answer:
When a rock is thrown straight upward, the net force on the rock is equal to zero at two specific points in its motion: at the highest point of its trajectory and at the instant it reaches its initial position.
1. Highest point of the trajectory: At the highest point, the rock momentarily comes to a stop before reversing its direction and falling back down. At this point, the gravitational force pulling the rock downward is balanced by the force exerted by the thrower in the upward direction, resulting in a net force of zero.
2. Initial position: When the rock reaches its initial position (the point from where it was thrown), its velocity becomes zero. At this moment, the gravitational force pulling the rock downward is once again balanced by the force exerted by the thrower in the upward direction, resulting in a net force of zero.
So, the net force on the rock is equal to zero at both the highest point of its trajectory and when it reaches its initial position.