Why do objects fall at the same speed in a vacuum?
In a vacuum, all objects fall at the same rate because gravity speeds up every object equally, about 9.8 m/s² on Earth. In air, drag slows lighter objects like feathers.
A heavier object feels a stronger pull of gravity, but it also has more inertia, meaning it is harder to get moving. The two effects cancel exactly, so every object accelerates at the same rate. Galileo argued this in the late 1500s, contradicting the ancient belief that heavy things fall faster.
On Earth, air resistance makes a big difference. A feather has a large surface compared with its weight, so the air slows it down a lot. A hammer is barely affected. Remove the air, and both fall together.
Proof on the Moon
In 1971, Apollo 15 astronaut David Scott dropped a hammer and a falcon feather at the same time on the Moon, which has almost no air. Millions of people watched on television as both hit the ground together, just as Galileo had predicted.
Did You Know?
A popular story says Galileo dropped balls from the Leaning Tower of Pisa to prove this. Historians are not sure the experiment really happened, but he did carry out careful tests with balls rolling down ramps.