The magnitude of the acceleration in the vertical direction for a projectile is 9.8 m/s². This is the acceleration due to gravity, and it acts on all objects near the Earth's surface, regardless of their horizontal motion.

Here's why:

* Gravity: The only force acting on the projectile vertically is gravity.

* Constant Acceleration: Gravity causes a constant acceleration downwards, which we approximate as 9.8 m/s².

Important Note: The projectile's horizontal motion doesn't affect its vertical acceleration. This is why the projectile will fall to the ground at the same rate as an object dropped straight down from the same height.