Here's why:
* Upward direction is typically positive: In most physics problems, we define the upward direction as positive and the downward direction as negative. This is a common convention, but it's important to note that it's a choice, not an inherent property of gravity.
* Gravity acts downwards: Since gravity pulls objects downwards, the acceleration due to gravity (g) is in the same direction as the negative direction of our chosen coordinate system.
Therefore, the acceleration due to gravity is often written as -9.8 m/s², indicating that it's acting downwards.
Important Note:
While the negative sign is a common convention, it's essential to understand the underlying physics. The direction of acceleration is dependent on the chosen coordinate system, and the actual direction of free-fall is always downwards regardless of the chosen sign convention.
