Motion and Newton's Laws
Before Newton: Two Kinds of Motion
For nearly two thousand years, physics followed Aristotle, who divided motion into two regimes. Earthly objects moved toward their "natural places" — stones fell, smoke rose — and required continuous pushing to move unnaturally, stopping when the push ceased. Heavenly bodies, meanwhile, were made of aether and moved eternally in perfect circles under entirely different principles. The heavens obeyed different physics than the earth; that assumption was so deep it was barely noticed as an assumption.
Everyday experience seemed to confirm it. Push a cart and it moves; stop pushing and it stops. Clearly, motion requires a cause that must keep acting. It took Galileo's experiments with inclined planes — and his brilliant thought experiments — to see the truth: friction was the hidden stopper. On an ideal frictionless surface, once moving, always moving.
The First Law: Inertia
Newton's first law states: an object at rest stays at rest, and an object in motion stays in motion at constant velocity in a straight line, unless acted upon by a net external force.
The revolutionary word is "stays." Uniform motion is not something that needs explaining — it is the default state of matter. What needs explanation is any change in motion: speeding up, slowing down, or curving. This inverted the old question. Aristotle asked "what keeps it moving?"; Newton answered "nothing does — what made it change?"
This principle is called inertia, and it explains experiences from daily life to spaceflight. When a car brakes hard, your body lurches forward not because some force pushes you but because no force has yet stopped you. Spacecraft coast for years with engines off precisely because there is almost nothing out there to slow them down.
The Second Law: F = ma
The second law quantifies change: the net force on an object equals its mass times its acceleration (F = ma). Acceleration — the rate of change of velocity — is proportional to the applied net force and inversely proportional to mass. The same push accelerates a shopping cart noticeably and a car barely, because mass measures resistance to acceleration.
This innocent-looking equation is arguably the most productive sentence ever written. Rearranged, it says: tell me all the forces acting on an object and I can compute its entire future motion by integrating acceleration over time. Forces are the inputs; motion is the output. Gravity near Earth's surface gives every object acceleration g ≈ 9.8 m/s² downward, regardless of mass — which is why, neglecting air resistance, a hammer and feather fall together. A spring stretched twice as far pulls twice as hard. Friction, drag, tension, thrust: identify the forces, apply F = ma, and nature computes the answer for you.
Note the precision of the terms. Force is measured in newtons (1 N = 1 kg·m/s²). Mass is not weight — weight is the gravitational force on a mass, which changes from Earth to Moon, while mass, the quantity of matter and resistance to acceleration, does not. An astronaut weighs one-sixth as much on the Moon but is just as hard to accelerate.
The Third Law: Action and Reaction
The third law states: for every action force there is an equal and opposite reaction force — never on the same body, always between two bodies. You press on the floor; the floor presses back on you equally. A rocket expels gas backward; the gas pushes the rocket forward — rockets work in empty space precisely because nothing external is required, only something to push against in reverse. A gun recoils; a swimmer pushes water backward to be pushed forward.
The third law also reveals why forces come in pairs and why "equal and opposite" doesn't mean cancellation: the two forces act on different objects. The Earth pulls you down; you pull the Earth up with equal force — the Earth just has vastly more mass, so your pull produces immeasurably tiny acceleration.
Unifying Heaven and Earth
The deepest achievement was conceptual. Newton showed that the same three laws plus one universal gravitational attraction account for the apple falling, the cannonball arcing, the Moon circling, the tides rising, and the planets wandering in ellipses as Kepler had observed. The Moon, Newton realized, is falling — perpetually — toward Earth; it simply moves sideways fast enough to keep missing. With his calculations, the ancient division between celestial and terrestrial physics collapsed into a single system.
This unification transformed humanity's picture of the cosmos. If heaven runs on the same mechanics as a thrown stone, then the universe is a machine whose behavior can be computed — Laplace later imagined an intellect that, knowing all positions and velocities, could see the entire future laid out. That deterministic vision dominated physics until quantum mechanics complicated it two centuries later.
And the laws still work. Engineers land rovers on Mars using Newtonian mechanics alone; discrepancies appear only at extreme speeds, masses, and scales, where relativity and quantum theory take over — refinements, not replacements, within everyday domains.