free fall

Free fall is what happens when gravity is the only force acting on an object. No air resistance, no friction, no buoyancy — just the steady pull of a planet or star. In free fall near the Earth's surface, every object accelerates downward at g ≈ 9.81 m/s², regardless of mass. A hammer falls at exactly the same rate as a feather, and a bowling ball falls at exactly the same rate as a golf ball. Aristotle thought otherwise; Galileo proved him wrong.

The reason everything falls the same is deep. The force of gravity on an object is proportional to its mass (F = mg), but the acceleration a force produces is inversely proportional to mass (a = F/m). The two mass terms cancel exactly, leaving an acceleration that depends only on the local gravitational field. This is called the equivalence of inertial and gravitational mass, and Einstein would later turn it into the foundation of general relativity.

On Earth, air resistance usually spoils the demonstration. A feather drifts, a parachute glides, a sheet of paper flutters — all because the surrounding air pushes back. Strip the air away, and the physics becomes visible. In 1971, astronaut David Scott dropped a hammer and a feather together on the surface of the Moon during the Apollo 15 mission. The footage is still striking: they touch the lunar dust at the same instant, because on the airless Moon, free fall is the only motion.

In a more technical sense, any body moving under gravity alone is in free fall, even if it is not falling downward. A spacecraft in orbit is in free fall — it is continuously falling toward the Earth, but its sideways motion is fast enough that the Earth curves away underneath it at the same rate. Astronauts inside the International Space Station are weightless because they, too, are in free fall, falling around the Earth together with the station.