11 December 2025 
4 minutes Once people thought… that motion requires force, and objects move because something pushes them every moment.
Today we understand… that objects keep moving at a constant speed and in the same direction as long as no external force acts on them. This is Newton’s first law, also called the law (or principle) of inertia — one of the three laws of motion that form the foundation of classical physics.
In April 1970, an explosion occurred aboard Apollo 13 as it headed for a lunar landing. After the astronauts managed to steer the damaged spacecraft onto a trajectory back to Earth, they had to shut down as many onboard systems as possible to conserve power — including the navigation computer. Mission commander Jim Lovell switched it off and told his crewmates, “We just put Sir Isaac Newton in the driver’s seat.” The line captures how central Newton’s laws of motion remain to physics, nearly 300 years after their publication.
Isaac Newton was born on December 25, 1642. Despite a difficult childhood — perhaps even because of it — he became a leading scientist. Among other things, he studied the nature of light, invented the reflecting telescope, and developed infinitesimal calculus, now known as differential and integral calculus — a key tool in mathematics, physics, and engineering.
In 1687 he published one of the most important science books in history, Mathematical Principles of Natural Philosophy (Philosophiæ Naturalis Principia Mathematica), better known by its short Latin title, Principia. In it, Newton presented the revolutionary idea that the same laws of nature govern the motion of all bodies — from an apple falling from a tree to Earth orbiting the Sun, and to every other astronomical body. At the heart of the book are his three laws of motion and the law of universal gravitation, which define the principles of that motion.
Newton’s first law states that a body at rest, or moving at constant velocity, will remain that way unless an external force acts to change its motion. The law’s key insight is that rest and uniform motion are physically equivalent: in both cases, the net force on the body is zero, meaning the forces balance each other out.
A ball lying on the grass will stay at rest until someone kicks it. After the kick, it should, in principle, keep moving in the same direction at the same speed until some other disturbance changes that. In practice, that isn’t what happens, because other forces act on the ball and affect its motion, including friction with the grass and air, and Earth’s gravity. If we kicked a ball in outer space, it would keep moving in the same direction at the same speed until another force stopped it — say, a collision with an alien spacecraft. The same is true for spacecraft launched into deep space: once their engines are shut down, they continue on at the same speed and in the same direction until the next engine burn.
