What are Newton's three laws of motion?
Quick answer
Isaac Newton formulated the three laws of motion, published in his book Principia in 1687. In short: (1) an object stays at rest or keeps moving unless a force acts on it; (2) force equals mass times acceleration (F = ma); (3) every action has an equal and opposite reaction.
The three laws
| Law | What it says | Everyday example |
|---|---|---|
| 1st: Inertia | An object at rest stays at rest, and a moving object keeps moving in a straight line at the same speed, unless a force acts on it | You lurch forward when a bus brakes suddenly |
| 2nd: F = ma | The force on an object equals its mass times its acceleration | Pushing an empty cart is easier than pushing a full one |
| 3rd: Action–reaction | For every action, there’s an equal and opposite reaction | A rocket pushes gas down, and the gas pushes the rocket up |
Law 1: Inertia
Objects “resist” changes in motion. On Earth, a rolling ball eventually stops because of friction and air resistance, which are forces. In space, with almost nothing to slow it down, a spacecraft keeps coasting for years, like the Voyager probes.
Law 2: F = ma
- F = force, measured in newtons (N)
- m = mass, in kilograms
- a = acceleration, in meters per second squared
Double the force and you double the acceleration; double the mass and you halve it. One newton is about the force needed to hold a small apple.
Law 3: Equal and opposite
Forces always come in pairs. When you jump, you push down on the ground and the ground pushes you up. When you fire a gun, it recoils backward.
Who was Isaac Newton?
Newton (1643–1727) was an English mathematician and physicist. He also described universal gravitation, explained how white light splits into colors, and co-invented calculus. The story that a falling apple inspired his ideas about gravity likely contains some truth, according to people who knew him.
Newton’s laws still guide engineering and space travel, from cars to missions to Mars. Einstein later refined them for very high speeds and strong gravity; see his view on imagination and knowledge.
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