The sign of the acceleration due to gravity is negative when an object is thrown upwards because we typically define upwards as the positive direction.

Here's why:

* Coordinate System: We establish a coordinate system to describe motion. In most cases, we choose upwards as the positive direction (+y) and downwards as the negative direction (-y).

* Gravity's Direction: Gravity always acts downwards towards the center of the Earth. Since we've defined downwards as negative, the acceleration due to gravity (g) is considered negative (-9.8 m/s²).

* Opposing Motion: When an object is thrown upwards, its initial velocity is positive (moving upwards), but gravity acts against that motion, pulling it downwards. This means the acceleration due to gravity acts in the opposite direction to the object's initial velocity.

To summarize:

* Upwards: Positive direction (+)

* Downwards: Negative direction (-)

* Gravity's Direction: Downwards (-)

* Therefore: Acceleration due to gravity (g) is negative (-)

This convention helps us understand the direction of motion and the effect of gravity on the object's velocity and position.