Here's why:
* Projectile motion is defined as the motion of an object thrown or launched into the air, where the only force acting on it is gravity.
* Gravity is a constant downward force that causes the object to accelerate vertically.
* Other forces, like air resistance, friction, or thrust, would alter the object's trajectory and make it deviate from true projectile motion.
In essence, a projectile is free-falling under the influence of gravity.
