Believed heavier objects fell faster than smaller ones

Natural motion in relation to the four-element model

He understood that the projectile's path is a combination of horizontal and vertical motion

A constant amount of force is needed to be applied to objects to keep them moving in horizontal motion.

Objects fall to the ground at the same rate (regardless of weight) unless

things like air resistance change the rate.

He defined the concept of force and its relation to motion

Theorized that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.