Final answer:
The acceleration in free-fall is constant at -9.80 m/s² and is downward, regardless of the object's velocity being zero at the highest point of its trajectory.
Step-by-step explanation:
The acceleration in free-fall is constant and directed downward even when the velocity at the top is zero. At the top of its trajectory, a free-falling object like a rock has zero velocity because it has momentarily stopped before starting to fall back down. However, the acceleration due to gravity on Earth is constant at approximately -9.80 m/s², which means that even at the top, where the velocity is zero, the rock is still experiencing this constant gravitational acceleration. This acceleration persists throughout the entire motion, whether the object is moving upward, at the peak of its ascent, or on the way down. It's important to note that the acceleration due to gravity acts in the same direction (downward) and with the same magnitude throughout the entire free-fall motion.