Final answer:
The positive direction defined when the y-component of an object's acceleration is 9.8 m/s² is typically upward. This direction is a conventional choice in physics, particularly for projectile motion and free-fall scenarios, though it ultimately depends on the chosen coordinate system.
Step-by-step explanation:
When an object is launched into the air and the y-component of its acceleration is 9.8 m/s2, it usually indicates the acceleration due to gravity, which acts downward. In physics, the conventional choice for coordinate systems in problems involving projectile motion is to take the upward direction as positive. Therefore, the direction defined as positive would be upward, meaning a = -g = -9.80 m/s2. However, the acceleration due to gravity is always directed down towards the Earth, so if down is defined as positive, then a = +g = +9.80 m/s2. The determination of positive direction depends on the initial choice of the coordinate system; for objects in free-fall or projectile motion, up is normally taken as positive for displacement, velocity, and acceleration.