ARISTOTLE'S FALLACY
The Greek thinker Aristotle (384 B.C 322 B.C.), held the view that if a body is moving, some external force is required to keep it moving. For example, an arrow shot from a bow keeps flying since the air behind the arrow keeps pushing it. For our understanding, the Aristotlelian law of motion may be stated as follows. An external force is required to keep a body in motion. But there is flaw in Aristotle's argument.
A ball rolling on the floor stops because an external force of friction between the ball and the floor and also resistance of air opposes the motion of the ball. If there was no opposing forces, the rolling ball would never stop.
Similarly, if there were no friction, there would be no need for the engine of a car to apply any force to keep the car in uniform motion.
The opposing frictional forces or viscous 2. H forces are always present in the natural world. This explains the practical experience that forces by external agencies are necessary to counter the frictional forces to keep bodies in uniform motion.
Now we understand where Aristotle went wrong. He stated this practical experience in the form of a basic law. To have correct law of nature for forces and motion, one had to imagine ideal conditions in which uniform motion took place with no opposing forces like frictional force, viscous force, air resistance. This is what Galileo did.