Here's why:

* Gravity: The primary force acting on a projectile in the vertical direction is gravity. Gravity constantly pulls the projectile downwards, causing a change in its vertical velocity.

* Acceleration due to gravity (g): This acceleration is approximately 9.8 m/s² near the Earth's surface. This means that the projectile's downward velocity increases by 9.8 meters per second every second it's in the air.

Important Note:

* While the vertical motion is accelerated, the horizontal motion of a projectile is generally considered constant velocity (neglecting air resistance). This is because there is no horizontal force acting on the projectile once it's launched.