From Aristotle to Galileo and Newton: The Idea of Inertia

Why do things stop?

Push a chair across the floor and let go. It stops almost at once. Kick a soccer ball across a lawn; it rolls for a while, then stops. Everyday experience seems to say: to keep something moving, you have to keep pushing it.

That idea feels so obvious that some of the greatest thinkers in history believed it for about two thousand years. Today we know it is wrong. This lesson tells the story of how scientists found out, and why the correct idea, inertia, is the foundation of Newton's laws.

Aristotle: motion needs a mover

The Greek philosopher Aristotle (384–322 BCE) divided motion into two kinds:

• Natural motion: heavy objects fall toward their natural place (the ground), and light things like smoke rise. He also taught that heavier objects fall faster.
• Violent (forced) motion: any other motion, such as a cart rolling along a road, needs a continuing push or pull. Remove the push and the motion stops.

Aristotle was a careful observer, and his ideas match daily life, where friction and air resistance are everywhere. But he mostly reasoned from observations; he did not test his ideas with controlled, measured experiments.

Galileo: measure it!

Galileo Galilei (1564–1642), an Italian scientist, insisted on measuring. Falling objects move too fast to time by hand, so he let balls roll down gentle ramps, which slowed the motion down. He timed them with a water clock (he weighed the water that flowed out during each roll).

He found that the total distances a ball rolls from rest after 1, 2, 3 and 4 equal time intervals are in the ratio 1 : 4 : 9 : 16 (the squares of the times), which means it is speeding up at a steady rate (constant acceleration, as you saw in the Kinematics unit). He argued that, apart from the effect of air, heavy and light objects fall with the same acceleration. (The famous story that he dropped two balls from the Leaning Tower of Pisa may be only a legend; his key evidence came from ramps and reasoning.)

Galileo's double ramp

Galileo then imagined a ball rolling down one ramp and up another. He noticed that, if friction were very small, the ball would climb to almost its starting height. Then he reasoned:

• Make the second ramp gentler. The ball still tries to reach the same height, so it rolls farther.
• Make the second ramp level. The ball can never reach its starting height, so it should roll forever.

His conclusion: a moving object does not need a force to keep moving. It slows down only because something, like friction, acts on it. This property of matter is inertia.

Newton: the laws of motion

Isaac Newton (1642–1727), an English scientist and mathematician, was born the year Galileo died. In 1687 he published the Principia, in which he:

• stated three laws of motion. The first law is Galileo's inertia stated as a general rule; the second connects force, mass and acceleration; the third says forces come in pairs;
• showed that the same gravity that makes an apple fall also keeps the Moon in orbit (universal gravitation, which you will study in SPH4U);
• used mathematics to make precise predictions that could be tested.

Newton wrote that if he had seen further than others, it was "by standing on the shoulders of giants". Galileo was one of those giants. Science is built step by step, each scientist testing and improving earlier ideas.

Newton's laws still work extremely well for everyday speeds and sizes. For objects near the speed of light, or for atoms, scientists later needed Einstein's relativity and quantum mechanics, but those theories agree with Newton's in everyday situations.