inertia (ĭnûrˈshə) [key], in physics, the resistance of a body to any alteration in its state of motion, i.e., the resistance of a body at rest to being set in motion or of a body in motion to any change of speed or change in direction of motion. Inertia is a property common to all matter. This property was first observed by Galileo and restated by Newton as his first law of motion, sometimes called the law of inertia. Newton's second law of motion states that the external force required to affect the motion of a body is proportional to that acceleration. The constant of proportionality is known as the mass, which is the numerical value of the inertia; the greater the inertia of a body, the less is its acceleration for a given applied force.