The law of inertia, formulated by Sir Isaac Newton, states that an object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity unless acted upon by a net external force. This principle is a fundamental aspect of classical mechanics and highlights the natural tendency of objects to maintain their state of motion. Inertia is directly proportional to an object's mass; the greater the mass, the greater the force needed to change its state. Understanding inertia is vital in areas such as physics, engineering, and various practical applications in everyday life.