The motion of objects fascinated Galileo throughout his life. When he wasn't conducting real-world experiments, he developed "thought experiments" to test his ideas about the way objects move, and why.

Centuries before Galileo revolutionized this area of physics, Aristotle concluded that objects of greater mass fall faster than those with less mass. This can certainly be observed with many of the objects we see around us every day. A dropped piece of paper, for example, drifts to the ground far more slowly than a falling soccer ball. However, Galileo suspected that Aristotle's universally accepted theory may not have been true. He predicted that differences in acceleration between different types of falling objects had nothing to do with either mass or Earth's gravitational force.

In several experiments and "thought experiments" Galileo recorded many hypotheses and observations and found evidence to support the possibility of a universal law governing the motion of all falling objects. Galileo probably dropped and threw countless objects from high places. Certainly, he thought about doing so under a variety of conditions.

Galileo also measured the time it took for pendulums of different weights to swing back and forth and the acceleration of balls rolling down inclined planes of different lengths. In the end, he concluded that the effect of gravity on earthly objects is the same, regardless of the mass of those objects. He argued that in the absence of other forces such as air resistance, all falling objects accelerate toward Earth at the same rate. In the centuries since Galileo's time, physicists have verified his conclusion and determined the rate of acceleration at Earth's surface to be 9.8 meters a second for every second the object is in free-fall.