Free fall

'Free fall' is when only gravity is acting on a body. For example, we do not say that a skydiver is exhibiting free fall, since the force of air resistance acts on them. Planets are in free fall though, but we will explain this. Since the time of Aristotle it was believed that the speed of a free falling object was proportional to its weight. It was also thought that this speed would be inversely proportional to the density of the medium the object was falling through. Aristotle also considered that the shape and orientation of an object impacts how it falls. 

Galileo attempted to test Aristotle's theory of free fall by dropping two spheres of different weights from the Tower of Pisa. Though this may have been more of a thought experiment than a real life event, it demonstrated that the lighter sphere fell at the same speed as the heavier one. This Aristotlean view of free fall was actually being challenged by thinkers at this time like Galileo. Objects of different mass accelerate at a constant rate.