free fall

Free fall is what happens to an object when gravity is the only thing acting on it — no air pushing back, no hand holding it, no engine. Drop a rock and, in the brief instant before air resistance matters much, it is in free fall. The surprising and beautiful fact, first nailed down by Galileo, is that in free fall every object speeds up at the very same rate, whether it is a feather or a cannonball — mass makes no difference at all.

Precisely, free fall is motion under gravity alone, so the object has a constant downward acceleration equal to g, the acceleration due to gravity (about 9.8 m/s^2 near Earth's surface). This means the constant-acceleration equations apply directly with a = g. Note that 'free fall' does not require moving downward: a ball thrown straight up is in free fall the whole time it is in the air, because gravity is still the only force on it — it just happens to be moving up (and slowing) for the first half.

The catch is the phrase 'gravity alone'. Real objects fall through air, and air resistance grows with speed, eventually balancing gravity so the object stops speeding up at its terminal velocity — that is why a feather flutters down slowly. So true free fall is an idealization we approach best for dense, compact objects over short drops, or exactly in a vacuum. The famous demonstration of a feather and a hammer landing together only works where there is no air, as astronauts showed on the airless Moon.