Aristotle's Fallacy

Before Newton, Aristotle (384-322 BC) proposed that a body needs a continuous external force merely to keep moving at constant velocity -- his reasoning being everyday observations such as a rolling ball, a flying paper plane, or a paper boat, each of which slows down and stops unless it is continually pushed. This is Aristotle's Fallacy: it sounds intuitive from casual observation, but it is wrong.

Galileo (1564-1642) gave the correct explanation, later formalised by Newton (1643-1727): every one of Aristotle's examples actually does have a continuously acting opposing force present -- it is just not obvious. The rolling ball is slowed by friction with the surface; the paper plane is slowed by the viscous drag of air; the paper boat is slowed by the viscous drag of water. None of these bodies is 'naturally' losing motion; they are being decelerated by real resistive forces that Aristotle failed to account for. Once such resistive forces are properly included, the true statement emerges: a body needs a net UNBALANCED external force only to CHANGE its state of motion (i.e., to accelerate); to overcome resistive forces and maintain constant velocity in the real world, an additional applied force is indeed needed, but this is fundamentally different from Aristotle's claim that force is needed to sustain motion i …