He imagined connecting a heavy weight to a light one and dropping them together. Now Aristotle’s theory makes two contradictory predictions:
The heavy weight should fall more slowly than normal, since the light weight now hinders it.
The heavy weight should fall more quickly than normal, since the two connected weights now form one very heavy object.
These can’t both be true, so Aristotle can’t be right. Instead the two weights must fall at the same rate.
At the end of the final Apollo 15 moon walk, astronaut David Scott dropped a geologic hammer and a feather simultaneously, to test this idea without air resistance. Both hit the surface simultaneously. Mission controller Joe Allen wrote that this was “reassuring, considering both the number of viewers that witnessed the experiment and the fact that the homeward journey was based critically on the validity of the particular theory being tested.”