Projectile Motion: A falling object was considered by Aristotle to be in "natural" motion, whereas a
projectile, such as an arrow shot from a bow, was thought to be in "violent" or "forced" motion. He
believed that forced motion could only come from a living agent and that a constant force is needed to
keep a projectile moving after it leaves the thrower. He thought that this continuous force was due to air
that was pushed out of the way by the projectile and swirled around behind the projectile to fill the
vacuum. Because Aristotle did not believe that an object could have two different motions at a time, in his
view, the trajectory of a projectile consisted of two phases: violent motion in the direction of the force that
gradually disappeared, followed by natural motion toward the center of the Earth.