Aristotle and Galileo, two different systems

Aristotle said the natural state of a body was to be at rest and it moved only if driven by a force or impulse. It followed that a heavy body should fall faster than a light one, because it would have a greater pull towards the earth.

Galileo disproved this view by dropping weights and observing that each body increased its speed at the same rate, no matter the weight. (Given the two bodies do not have much air resistance). 

In his experiment, as a body rolled down the slope it was always acted on by the same force (its weight), and the effect was to make it constantly speed up. This showed the real effect of a force is always to change the speed of a body, rather than just to set it moving. 

It also mean that where near a body is not acted on by any force, it will keep on moving in a straight line at the same speed (newton's first law).