It’s a well-known fact that a feather will fall slower than a bowling ball due to air resistance. However, what happens if you remove the air and drop both objects in a vacuum? Surprisingly, they will fall at the exact same speed. This might seem counterintuitive, but it’s actually a result of a fundamental principle of physics known as the equivalence principle. In this article, we’ll explore why bowling balls and feathers (in general, heavy objects and light objects) fall at the same speed in a vacuum and the implications of this principle on our understanding of gravity.

## The role of air resistance in falling objects

It’s one of the basic Newton laws: how fast something falls due to gravity is determined by a number known as the “acceleration of gravity”, which is 9.81 m/s2 at the surface of Earth. The acceleration of gravity, shortly “a” (in fact, for free fall, “g” – short for gravitational acceleration) means that in one second, any object’s downward velocity will increase by 9.81 m/s because of the Earth’s gravity. Gravity accelerates everything at exactly the same rate.

This also means a heavy object like a bowling ball and a lightweight object like a feather should fall down at the same speed, regardless of their shape. But we see this phenomenon very rarely in our daily lives. The reason is air resistance. It affects the objects’ falling speed: lighter objects fall slower than heavy objects because of air resistance. That’s why people (including Aristotle) thought that heavier objects fall faster for thousands of years.

It seems Galileo Galilei was the first person to notice that different things fall at the same rate. According to a biography by Galileo’s pupil Vincenzo Viviani, in 1589 the Italian scientist had dropped two balls of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass. But there was really no explanation of why until Sir Isaac Newton developed three physical laws that together laid the foundation for classical mechanics now known as Newton’s laws of motion.

## Falling in a vacuum: do things really fall at exact same speed?

In the amazing video from the BBC below, physicist Brian Cox visits the Space Power Facility in Ohio, a vacuum chamber built by NASA in 1969. It stands 122 feet (37 meters) high and 100 feet (30 meters) in diameter, enclosing a bullet-shaped space. With a volume of 22,653 cubic meters, it’s the largest vacuum chamber in the world.

The facility was originally commissioned for nuclear-electric power studies under vacuum conditions but was later decommissioned. Recently, it was recommissioned for use in testing spacecraft propulsion systems. Recent uses include testing the airbag landing systems for the Mars Pathfinder and the Mars Exploration Rovers, Spirit, and Opportunity, under simulated Mars atmospheric conditions.

Cox makes a “falling in a vacuum” experiment: first, he drops a bowling ball and a feather under normal conditions (with air in the chamber), and then he repeats it in a vacuum after all the air has been sucked out of the chamber. It’s really worth watching.

## Video: Apollo 15 astronaut Dave Scott proves Galileo was right

At the end of the last Apollo 15 moonwalk (August 2, 1971), Commander David Scott held out a geologic hammer and a feather and dropped them at the same time. Because they were essentially in a vacuum, there was no air resistance and the feather fell at the same rate as the hammer, as Galileo had concluded hundreds of years before.

## Brian Cox

Brian Edward Cox OBE, FRS (born 3 March 1968) is an English physicist who serves as a professor of particle physics in the School of Physics and Astronomy at the University of Manchester.

Cox is best known to the public as the presenter of science programs, especially the Wonders of… series, and for popular science books, such as Why Does E=mc2? and The Quantum Universe. He has been the author or co-author of over 950 scientific publications.

Cox has been described as the natural successor for BBC’s scientific programming by both David Attenborough and Patrick Moore. Before his academic career, Cox was a keyboard player for the British bands D:Ream and Dare.