In order to do this he had to learn how to measure distance and time very accurately. If an object changes speed or direction then it is accelerating. Slowing down is a form of acceleration. Because the longer it falls the faster it falls, a penny thrown off the Empire State building can go through a car and kill you. If you jump off a building that is 30 feet high, then you will fall for 1 sec, and hit the ground going 22 mph.
People still believe that heavy objects fall faster than lighter ones.