Projectile Motion

Before Galileo, the general thought about projectiles was that they traveled based on the theory of impetus or momentum. This theory stated that a projectile followed a straight line until it lost its impetus, at which point the object would fall straight to the ground.

After closer observation, it was discovered that projectiles didn't follow paths consisting of two straight lines, but rather a curved path. However, the exact mathematics of a projectile's curve was still unknown.

Galileo was the first to properly describe projectile motion as consisting of separable horizontal and vertical components. After close observation, Galileo determined that the only vertical force acting on a projectile was gravity (9.81 meters/second^2). He also observed that the horizontal motion of a projectile was constant and followed his law of inertia. The law of inertia states that an object will maintain its state of motion as long as no outside force acts upon the object. Galileo then explained that when the vertical and horizontal motions are combined, they form a mathematical curve known as a parabola.

Based on these laws, it can be seen that the horizontal motion of a projectile is indeed constant, and vertical motion is controlled by gravity.