The English physicist Brian Cox tests how a heavy object (a bowling ball) and a lightweight object (feather) fall in a vacuum – if they fall at the exact same speed or not.

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 shapes. 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: Galileo's Leaning Tower of Pisa experiment
In the presence of the Grand Duke, Galileo Galilei makes the Leaning Tower of Pisa experiment and shows the audience that all objects fall at the same speed – regardless of their weights. Painting by Luigi Catani ( 7 November 1762 – 17 December 1840), an Italian painter. Photo from: Museo Galileo website

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.

Falling in a vacuum: do things really fall at exact same speed? Brian Cox visits the world’s biggest vacuum chamber
Brian Cox visits NASA’s Space Power Facility in Ohio to see what happens when a bowling ball and a feather are dropped together under the conditions of outer space.

Brian Cox

Brian Edward Cox OBE, FRS (born 3 March 1968) is an English physicist who serves as 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.

Brian Cox’ Twitter: @ProfBrianCox
Official website:


M. Özgür Nevres
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