The acceleration of a ball thrown straight up at its highest point is -g, where 'g' is the acceleration due to gravity (approximately 9.8 m/s²).

Here's why:

* Gravity is constant: Throughout the entire flight of the ball, gravity acts on it consistently, pulling it downwards.

* Velocity at the highest point: At the highest point, the ball momentarily stops moving upwards before it starts falling back down. Its vertical velocity is 0.

* Acceleration is independent of velocity: While the velocity changes, the acceleration due to gravity remains constant.

Therefore, even though the ball's velocity is zero at the highest point, it's still accelerating downwards at -g. The negative sign indicates that the acceleration is in the opposite direction to the initial upward motion.