Newton’s second law of motion . Newton’s third law of motion . Conservation of momentum . Equilibrium of a particle .
Common forces in mechanics . Circular motion . Solving problems in mechanics Summary Points to ponder Exercises . ARISTOTLE’S FALLACY The question posed above appears to be simple.
However, it took ages to answer it. Indeed, the correct answer to this question given by Galileo in the seventeenth century was the foundation of Newtonian mechanics, which signalled the birth of modern science. The Greek thinker, Aristotle ( B.C– B.C.), held the view that if a body is moving, something external is required to keep it moving. According to this view, for example, an arrow shot from a bow keeps flying since the air behind the arrow keeps pushing it.
The view was part of an elaborate framework of ideas developed by Aristotle on the motion of bodies in the universe. Most of the Aristotelian ideas on motion are now known to be wrong and need not concern us. For our purpose here, the Aristotelian law of motion may be phrased thus: An external force is required to keep a body in motion . Aristotelian law of motion is flawed, as we shall see.
However, it is a natural view that anyone would hold from common experience. Even a small child playing with a simple (non-electric) toy-car on a floor knows intuitively that it needs to constantly drag the string attached to the toy- car with some force to keep it going. If it releases the string, it comes to rest. This experience is common to most terrestrial motion.
External forces seem to be needed to keep bodies in motion. Left to themselves, all bodies eventually come to rest. What is the flaw in Aristotle’s argument? The answer is: a moving toy car comes to rest because the external force of friction on the car by the floor opposes its motion.
To counter this force, the child has to apply an external force on the car in the direction of motion. When the car is in uniform motion,