The inertia of an object is a measure of its resistance to a change in the state of its motion. It is solely dependent on the mass of the object, with more massive objects having larger inertia and a greater tendency to resist changes to their motion.

Originally stated by Galileo as ‘in the absence of a force, a moving object will continue to move’, the concept of inertia was refined and re-defined by Newton in his first law of motion:

Every object continues in a state of rest, or uniform motion in a straight line, unless acted upon by external forces.

Newton’s first law is also known as the ‘law of inertia’.