When a ball is thrown vertically upward, its velocity decreases until it reaches the highest point of its trajectory. At this point, the ball momentarily stops before starting to fall back down due to the force of gravity.
The acceleration experienced by the ball at the highest point is equal to the acceleration due to gravity, denoted as 'g'. Near the surface of the Earth, the value of acceleration due to gravity is approximately 9.8 m/s².
Therefore, at the highest point of its trajectory, the acceleration of the ball is approximately 9.8 m/s² in the downward direction (opposite to the direction of the ball's motion).