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 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.