The fall of a big rock and small rock near the Earth' surface is a free fall. Both rocks will fall at the same acceleration of gravity. If both rocks get started at the same height, they will reach the ground at the same time. This fact was proved by Galileo. Galileo was sure that air resistance dominated the gravitational force for very light objects, such as papers and feathers that have large surface areas. Without air resistance, all objects will fall with the same constant acceleration.