Push a book across a table and it stops. Kick a ball and it rolls, slows and comes to rest. For about two thousand years the natural conclusion seemed obvious: moving things stop unless something keeps pushing them. The first great lesson of physics was that this obvious conclusion is wrong.

Aristotle's common sense

The Greek philosopher Aristotle, writing in the fourth century BC, taught that the natural state of an earthly object is rest, so it keeps moving only while something pushes it. A cart moves while the ox pulls it; stop pulling and it halts. He also held that heavier objects fall faster than lighter ones. These ideas matched everyday experience so well that they dominated learning in Europe and the Islamic world for centuries. Some critics saw problems. Why does an arrow keep flying after it leaves the bow? The sixth century scholar John Philoponus and, later, the fourteenth century Parisian teacher Jean Buridan proposed that a thrown object carries an "impetus" given to it by the thrower, which slowly wears off.

Galileo and inertia

Galileo Galilei, working in Italy in the early 1600s, saw that friction and air resistance hide the real behavior of moving things. He imagined a ball rolling down one smooth ramp and up another. The ball climbs to nearly its starting height. Lower the slope of the second ramp and the ball rolls farther to reach that height. Make the second surface flat and perfectly smooth, and the ball would never stop. The modern name for this is inertia: the tendency of an object to keep its current state of motion, at rest or moving steadily in a straight line, unless a force acts on it.

Galileo also argued that without air resistance, heavy and light objects fall at the same rate. The story that he dropped balls from the Leaning Tower of Pisa is probably a legend, but the result has been confirmed many times. In 1971 the Apollo 15 astronaut David Scott dropped a hammer and a falcon feather on the airless Moon, and they landed together.

Newton's three laws

In 1687 Isaac Newton published the Principia, which set out three laws of motion.

The first law restates inertia: an object stays at rest or keeps moving in a straight line at constant speed unless an unbalanced force acts on it.

The second law connects force to change. An unbalanced force changes an object's motion, and the same force produces a smaller change in a more massive object. Force causes acceleration, meaning any change in speed or direction, not motion itself. A car cruising at a steady speed on a straight road has no net force on it: the engine's push exactly balances friction and air drag. Mass is the amount of matter in an object and its resistance to being accelerated, while weight is the force of gravity pulling on that mass. An astronaut's mass is the same on the Moon as on Earth, but their weight there is about one sixth as much.

The third law is the most misunderstood. It says that forces come in pairs: when one object pushes on a second, the second pushes back on the first with an equal force in the opposite direction. A rocket pushes hot gas out of its nozzle, and the gas pushes the rocket the other way. So a rocket's own exhaust pushes it forward, and it needs no air to push against. In 1920 an editorial in The New York Times mocked the rocket pioneer Robert Goddard for thinking a rocket could work in a vacuum. The paper printed a correction in July 1969, as Apollo 11 headed for the Moon.

Momentum

Newton's laws lead to one of physics' great bookkeeping rules. Momentum is mass multiplied by velocity: a slow truck and a fast bullet can both carry a lot of it. In a closed system, where no outside force acts, the total momentum stays the same. This is conservation of momentum. When two skaters standing together push apart, they glide off in opposite directions, and the lighter one moves faster.

Worked example: the seat belt

A car traveling at 50 kilometers per hour hits a wall and stops in a fraction of a second. By the first law, an unbelted passenger keeps moving forward at 50 kilometers per hour until something stops them: the steering wheel, the dashboard or the windshield. That impact lasts only an instant, so the force on the body is enormous.

A seat belt and an airbag apply the stopping force earlier and spread it over a longer time and a wider area of the body. The passenger's total change in motion is the same, but because it happens more gradually, the force at any moment is much smaller. The crumple zone at the front of a modern car works the same way: it is designed to collapse, stretching out the time of the crash. Newton's laws, applied with care by engineers, are a large part of why road deaths per kilometer driven have fallen so far in many countries since the 1960s.