Final answer:
At the top of its trajectory, the rock has zero velocity but is still under the influence of gravity, which exerts a downward force, represented by a constant acceleration of -9.80 m/s².
Step-by-step explanation:
When a rock thrown straight upwards gets to the exact top of its path, its velocity is momentarily zero; however, this does not mean that the net external force acting on the rock is zero. Even at the top of its trajectory, the rock is still subject to Earth's gravity, which exerts a constant downward force. The acceleration due to gravity is -9.80 m/s² throughout the entire motion of the rock, both on the way up and on the way down.
It's crucial to understand that the zero velocity at the top of the path is merely an instantaneous condition in the rock's motion. The constant acceleration due to gravity ensures that, after reaching this point, the rock will begin to fall back towards the ground, gaining speed at the same rate it lost speed on the way up. This demonstrates that the force of gravity is acting on the object throughout its entire journey, influencing its speed and direction of travel.