Galilean Inertia
Inertia is the property of matter that resists changes in motion. An object in motion continues moving at constant speed in a straight line unless a force acts on it. An object at rest remains at rest unless a force acts on it. This is Newton's first law, but the principle was established by Galileo Galilei in the early seventeenth century — which is why it bears his name.
Before Galileo, the Aristotelian view held that objects naturally come to rest; motion requires a continuous cause. A rolling ball slows and stops not because of friction (that was not the explanation) but because motion was thought to be inherently unstable. Galileo showed by experiment and argument that this was backwards. A ball on a perfectly smooth horizontal plane would keep rolling forever — it is the friction and air resistance that stop it. Remove those, and there is no reason for motion to end.
Why It Was Radical
The inertia principle is subtle because the resistive forces in everyday experience (friction, air resistance) are so ubiquitous that "objects slow down without a push" looks like a natural law. Galileo's insight was to idealize: imagine the friction away, and ask what would happen. The answer — perpetual motion at constant velocity — is not obvious from everyday experience.
This idealization technique is characteristic of mathematical physics: isolate one factor by imagining all others removed, predict the behavior in that ideal case, then add back the complications one by one. Newton extended the technique systematically.
All Things Fall at the Same Rate
Galileo also established that all objects fall at the same rate in a vacuum, regardless of their mass. (Legend attributes a demonstration from the Leaning Tower of Pisa; the actual evidence was careful inclined-plane experiments that slowed the fall enough to time.) A heavy cannonball and a light musket ball, dropped simultaneously, hit the ground at the same moment. This contradicted Aristotle, who held that heavier objects fall faster.
The equality of gravitational and inertial mass — the fact that the same property (mass) that makes an object hard to accelerate also makes it attracted to the Earth in exact proportion — turns out to be one of the deepest facts in physics, later elevated to the equivalence principle at the heart of general relativity.
Connections
- universal-gravitation — Newton unified Galileo's free-fall discovery with planetary motion
- equivalence-principle — inertial mass = gravitational mass; the deepest implication of Galileo's result
- general-relativity-gravity — Einstein built his theory of gravity on the equivalence principle