Old (and
incorrect) idea of what
inertia is. It was once believed that once an
object started to
rise, it would only rise until it ran out of impetus, in which case it would immediately fall
straight to the ground.
Unfortunately, this neglected the fact that projectiles travel in a parabolic curve (because of the constant acceleration due to gravity), and so it was disproven by Newton because it was Newton's laws that introduced inertia, an accurate description of projectile motion.