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$g$ refers to gravitational acceleration due to gravitational force. When an object moves upwards, it moves against gravity, so it decelerates at a rate of $9.81\;\mathrm{ms^{-2}}$, meaning it accelerates at a rate of $-9.81\;\mathrm{ms^{-2}}$. When moving downwards, it moves with gravity, so it accelerates at a rate of $9.81\;\mathrm{ms^{-2}}$.