Do heavy things fall faster than light things?
― Galileo tested it on a ramp, not the Leaning Tower

"Heavier things fall faster." This idea was believed to be true for almost 2000 years. In reality, though, without air, heavy and light objects fall at exactly the same time. The people who proved it did so with a clever trick: they slowed the fall down so it could be measured.

There are only two reasons things of different weights fall together

Obvious as it sounds now, this was hard to prove. A fall takes only an instant, and people in earlier centuries had no clock precise enough to measure whether two things landed "at the same time."

Why was "heavier falls faster" believed for 2000 years?

Aristotle, in ancient Greece, held that heavier things fall faster. Compare a stone with a feather and that certainly seems true — because in everyday falls, air is always mixed into the picture.

In 1586, the Dutch scientist Simon Stevin and his colleague Jan Cornets de Groot recorded dropping two lead balls from a church tower, one ten times heavier than the other. The height was said to be about 9 metres, and the two balls were reported to hit the board below with what sounded like a single thud.

Italy's Galileo Galilei reached the same conclusion. The famous story of him dropping balls from the Leaning Tower of Pisa comes from a biography written by his student Vincenzo Viviani after Galileo's death. It appears nowhere in Galileo's own writings, and historians still debate whether it really happened.

Galileo slowed the fall down using a ramp

What Galileo actually documented was a far more painstaking experiment. He carved a straight groove into a board, tilted it, and rolled a bronze ball down it. On a slope, the ball still obeys the same rule, but falls slowly enough to observe.

He measured time by letting water trickle through a narrow tube from a container, then weighing the water that collected. This let him compare how far the ball travelled in each equal interval of time.

The result is shown in Figure 1. Taking the distance covered in the first interval as 1, the next interval covered 3, then 5, then 7 — increasing by odd numbers. The running totals from the start were 1, 4, 9, 16 — the square of the time elapsed. This pattern held no matter how heavy the ball was.

Galileo also argued against Aristotle with a thought experiment. Tie a heavy stone and a light stone together with a string and drop them. What happens?

If the light stone drags on the heavy one, the combined pair should fall slower than the heavy stone alone. But the pair together is also heavier than the heavy stone alone — so if "heavier falls faster" is true, the pair should also fall faster. Slower and faster at once is a contradiction, so he argued the original idea itself must be wrong.

How big a difference does air actually make?

Galileo was well aware of air's effect. In his book, he wrote that dropping a large iron ball and a small one from a height of about 60 metres produced a landing gap of only about two finger-widths. Air does cause some difference, he argued, but nowhere near the "difference proportional to weight" that Aristotle claimed.

The gap grows largest for things with a lot of surface area relative to their weight — paper, feathers, raindrops. A raindrop reaches a certain speed and then stops accelerating, falling at a constant rate from then on (explained in detail in why raindrops don't hurt).

In 1971, Apollo 15 astronaut David Scott dropped a hammer and a bird feather at the same moment on the Moon. As shown on the right of Figure 2, the two fell side by side and hit the ground together. Galileo's conclusion was confirmed live, on television.

Summary

Heavier objects are pulled more strongly, but they're also proportionally harder to move, so falling speed doesn't depend on weight. Paper and feathers lag behind only because air is braking them. Galileo found this rule by slowing falls down on a ramp and timing them with a water clock.

Weight does not decide how fast something falls.
What undid a 2000-year-old assumption was a ramp and a water clock.

For how weight itself was ever measured, see how the Earth's weight was measured; for why your body feels lighter while falling, see why you feel lighter in an elevator.

※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding, not precise values. Accounts of the historical experiments vary between sources in their details.