Newton’s Three Laws of Motion

Abstract

1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it: this is the law of inertia. 2. Under Newton, F = ma, but under Aristotle F = mv, where v is the velocity. (F = force, m = mass, a = acceleration). According to Aristotle there is only a velocity if there is a force, but according to Newton an object with a certain velocity maintains that velocity unless a force acts on it to cause an acceleration (that is, a change in the velocity). Aristotle's view seems to be more in accord with common sense, but that is because of a failure to appreciate the role played by frictional forces. Once account is taken of all forces acting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that are found to be in accord with the observations. 3. For every action there is an equal and opposite reaction. This is explained by what happens if we step off a boat onto the bank of a lake: as we move in the direction of the shore, the boat tends to move in the opposite direction (leaving us facedown in the water, if we aren't careful!).