Galileo and the Concept of Inertia

Perhaps Galileo's greatest contribution to physics was his formulation of the concept of inertia: an object in a state of motion possesses an "inertia" that causes it to remain in that state of motion unless an external force acts on it. In order to arrive at this conclusion, Galileo had to abstract from what he, and everyone else, saw.

The Hidden Role of Friction

Most objects in a state of motion do not remain in that state of motion, as illustrated in the diagram at the top of the page. For example, a block of wood pushed at constant speed across a table quickly comes to rest when we stop pushing. Galileo, by virtue of a series of experiments (many with objects sliding down inclined planes), realized that this was because there was a hidden force, friction, acting between the surface and the object.

As we push a block of wood across a table, there are two opposing forces that act: the force associated with the push, and a force that is associated with friction and that acts in the opposite direction. This is illustrated in the figure shown above.

Galileo realized that as the frictional forces were decreased (for example, by placing oil on the table) an object given an initial push would move further and further before stopping. From this he abstracted a basic form of the law of inertia: if the frictional forces could be reduced to exactly zero (not possible in a realistic experiment, but it can be approximated to high precision) an object set in motion at constant speed across a frictionless surface of infinite extent will continue at that speed forever, unless a force acts on it at a later time. This animation illustrates the law of inertia.