Final answer:
The acceleration at the highest point of a ball thrown straight up without air resistance is 9.8 m/s² downwards. With air resistance, the return speed is less than the initial speed and the maximum height reached is lower.
Step-by-step explanation:
When a ball is thrown straight up with no air resistance, the acceleration at its highest point is still directed downward towards the center of the Earth and has a magnitude of approximately 9.8 m/s2. This acceleration due to gravity remains constant throughout the ball's flight, whether going up, at the top, or coming down. At its highest point, the ball's velocity is momentarily zero as it changes direction from up to down, but the acceleration does not change.
If air resistance were not negligible, the speed upon return would be less than its initial speed, because air resistance would remove energy from the system as the ball ascends and descends, slowing it down. Additionally, the maximum height reached by the ball would be lower than it would be in a vacuum because the air resistance would slow the ball more quickly as it rises, causing it to reverse direction at a lower altitude than it would without air resistance.