According to the traditional account, to refute the Aristotelian notion that heavier objects fall faster than light ones, Galileo performed an experiment from the top of the leaning tower of Pisa. He dropped two spheres of different weight and observed that both hit the ground at the same time. Galileo correctly reasoned that when an object falls more slowly, it is due to air resistance. An object moving solely under the influence of gravity is said to be in free fall; near the surface of the Earth such an object falls at an acceleration of 9.8 .