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Hello, why is the acceleration -9.8 m/s^2 when after the ball reaches the top, the acceleration is +9.8m/s^2?

Hi choianchoi, I've writen the acceleration as -9.8m/s^2 in the formula, and it's important to stay consistent once you've chosen a coordinate system. That is to say that if down is chosen to be negative, as is often the case, then it has to be considered negative for the entire question. Since we've chosen down as negative, the acceleration due to gravity can never be positive, regardless of where the ball is in it's trajectory.

Best wishes,
Mr. Dychko