Free Fall.- It's an objectís motion towards the ground, with an almost constant acceleration, and dismissing the air resistance.
The difference between the angle of this force is so small, that these forces are considered parallels.
Galileo Galilei discovered that all objects fall at the same speed, regardless of their mass. When he proposed this, he contradicted Aristotleís principle. This principle says that the speed in which an object falls is proportional to its weight, but it's completely wrong.
This was proofed when the man landed in the moon, when the astronaut drops a feather an a hammer at the same time and they reach the ground at the same time.