Here's why:
* Gravity is Constant: The only force acting on the rock throughout its flight is gravity. Gravity pulls objects towards the center of the Earth with a constant acceleration.
* Top of Trajectory: At the very top of its trajectory, the rock momentarily stops moving upwards. However, gravity is still acting on it. This means it's about to start moving downwards.
* Direction of Acceleration: Acceleration is a vector quantity, meaning it has both magnitude (strength) and direction. The acceleration due to gravity always points downwards towards the Earth.
Important Note: Even though the rock's velocity is zero at the top of its trajectory, its acceleration is not zero. Acceleration describes the rate of change of velocity, and gravity continues to change the rock's velocity even at the highest point.
