5.2 Aristotle’s fallacy

Is an external force required to keep a body in uniform motion? Aristotle and many more scientists were even mistaken about it. Aristotle explained that objects velocities were proportional to the forces exerted on them. It was true for a behaviour shown by sliding objects but according to it heavier objects should fall faster than lighter objects. The people accepted this theory because they could not find a better answer to this question. So this theory remained unparalleled. Then Italian astronomer, mathematician, and physicist Galileo Galilei (1564-1642) gave a logical analysis of this question. According to him if a person is stationary, he/she will remain stationary if he/she is moving in some particular direction with a steady velocity, he/she will remain moving with the same velocity in that direction until some external influence is acted upon it. This he called inertia. Galileo Galilei also devised experiments to resolve the conflict between Aristotle’s theory and observations of the world around him. He determined that falling objects under the action of gravity alone fall at the same rate.

So that way he could resolve the ambiguity. He said Aristotle assumed the requirement of an external force to keep the body in motion, is actually due to the presence of certain dissipative forces present around us, Like internal resistance of air, frictional forces, etc.

So one has to overcome these forces to maintain their motion. In the absence of such forces (dissipative forces), one needs no external force to continue in the same state of rest or of uniform motion.

Coming back to the example quoted above, we talk about the toy car which has to be dragged as long as we want it to be in motion. Once you leave the string loose, the car comes to rest. Similarly, you roll a ball on the floor. It eventually slows down and then comes to rest. If you want the car or the ball to move again, you need to pull the string or roll the ball again. In other words, you need to apply force on them.

So Aristotle made observations from practical experiences and came to the conclusion that an external force is required to keep a body in uniform motion. This is known as Aristotle’s fallacy.