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Published: 16 November 2020

• mechanics
• space

This week we’re taking a look at an ever-popular demonstration of the laws of motion: Free Fall in Vacuum. You can see it presented in this new video starring Logan Anbinder.

A heavy disc and a lightweight feather are contained in a long glass tube. While the tube is full of air, when you flip it over the disc drops quickly to the bottom, and the feather floats slowly downwards. Once most of the air has been pumped out, however, the two fall together and hit the bottom at the same time.

When no other forces interfere, the two objects experience the same acceleration from gravity. While there is air in the tube, however, air resistance slows both objects, but the feather more so than the disc due to its lighter mass compared to its surface area. This is an excellent example of why, when solving physics problems, we need to identify all of the forces involved in a system and determine their effects.

This demonstration is a classic way of explaining the nature of free fall and gravitational acceleration. Perhaps one of the most dramatic presentations of it came in 1971 when astronaut David Scott carried out this experiment with a hammer and feather on the Moon! Without air, the feather and hammer dropped together to the lunar soil. You can see the video in a NASA archive website here, hosted by Goddard Space Flight Center: https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html