Final answer:
At the highest point of its trajectory, a baseball thrown straight up has an acceleration of approximately 9.8 m/s² downward due to gravity.
Step-by-step explanation:
When a baseball is thrown straight up and reaches its highest point, its acceleration is that of gravity, which is approximately 9.8 m/s² downward. This occurs because, at the highest point, the velocity of the baseball is momentarily zero, but it is still subjected to the constant acceleration due to gravity.
No matter the path of the projectile, the acceleration due to gravity is constant and points downward, which is why, neglecting air resistance, the ball will experience this constant acceleration at all points throughout its trajectory.