Do heavy things fall faster than light ones?
― Galileo tested it on a ramp, not from a tower

"Heavier things fall faster." For nearly 2,000 years, this was taken as fact. But in reality, without air in the way, heavy and light objects fall at exactly the same rate. The people who proved this had a clever trick: slow the fall down enough to measure it.

There are only two reasons objects of different weights fall together

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

Why was "heavier falls faster" believed for 2,000 years?

Aristotle, in ancient Greece, held that heavier objects fall faster. Compare a stone and a feather, and it certainly looks that way — because every fall we see in daily life has air mixed into it.

In 1586, the Dutch engineer Simon Stevin and his friend 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 strike the board below with what sounded like a single thud.

Italy's Galileo Galilei reached the same conclusion. The famous story of Galileo dropping balls from the Leaning Tower of Pisa comes from a biography written by his student Vincenzo Viviani after Galileo's death. It doesn't appear in Galileo's own writings, and historians still debate whether it actually happened.

Galileo slowed the fall down using a ramp

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

He measured time by letting water trickle through a thin 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. If the distance covered in the first interval is called 1, the next interval covers 3, then 5, then 7 — increasing in odd numbers. The running totals from the start are 1, 4, 9, 16 — the square of the elapsed time. 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 object should fall slower than the heavy stone alone.

But the combined object is also heavier than the heavy stone alone. If "heavier falls faster" is true, it should also fall faster. Slower and faster at once is a contradiction — so, he argued, the original premise must be wrong.

How much difference does air actually make?

Galileo was well aware of air's effect. In his writings, he noted that dropping a large iron ball and a small one from 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 actually stops accelerating once it reaches a certain speed and falls at a constant rate after that (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

A heavier object is pulled down harder, but it's also proportionally harder to move — so its falling speed doesn't depend on weight at all. Paper and feathers lag behind only because air brakes them. Galileo uncovered this rule by slowing falls down on a ramp and timing them with a water clock.

Weight does not determine how fast something falls.
What undid a 2,000-year-old assumption was a ramp and a water clock.

For how weight itself was ever measured, see how the weight of the Earth was measured; for why your body feels light 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 of the underlying mechanism. Accounts of historical experiments vary in detail between sources.