Lesson 1 of 11 · 7 min

Aristotle's fallacy and the law of inertia

NCERT § "Aristotle's Fallacy" and § "The Law of Inertia"

On a level stretch the driver switches the engine off at 10 m/s. The bus rolls on for about 100 m before it stops. Did it stop because nothing was pushing it any more?

The story this chapter follows: Rohan's match day

Rohan weighs 50 kg and carries a 5 kg kit bag. On match day he takes a city bus to the cricket stadium, fields in the outfield, rides the stadium lift and drags his bag home. Every force in this chapter turns up somewhere on that trip. Take g = 10 m/s² unless a sim says otherwise (the sims use 9.8).

The lesson in notes

In short

Aristotle held that a body needs a continuing external force just to keep moving. This is wrong; in daily life an applied force is needed only to overcome opposing forces such as friction and air drag.

Galileo reasoned from motion on inclined planes: a body speeds up going down a slope and slows going up, so on a smooth horizontal plane it should neither speed up nor slow down.

His double-inclined-plane argument: a ball rolled down one smooth slope climbs the other to nearly the same height; making the second slope flatter makes it travel farther, and with a horizontal second plane it would go on forever.

Conclusion: a state of rest and a state of uniform straight-line motion are equivalent; no net force is required to maintain either.

Inertia is the resistance of a body to a change in its state of rest or uniform motion. Mass is the quantitative measure of inertia.