Why does gravity make objects accelerate at the same rate in a vacuum?

In a vacuum, the only significant force acting on a falling object is gravity, so differences caused by air resistance disappear. Newton’s second law explains why the acceleration is the same: although gravity pulls more strongly on a larger mass, that larger mass also resists acceleration more strongly by the same factor. The result is that, near Earth’s surface, all objects accelerate at about 9.8 m/s² regardless of mass. This was famously demonstrated in 1971 when astronaut David Scott dropped a hammer and a feather on the Moon during Apollo 15, where there is essentially no air. Galileo’s long-ago insight was later confirmed with precision experiments in vacuum chambers.