Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes its velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newton in his first law of motion (also known as The Principle of Inertia).^[1] It is one of the primary manifestations of mass, one of the core quantitative properties of physical systems.^[2] Newton writes:^[3][4][5][6]

LAW I. Every object perseveres in its state of rest, or of uniform motion in a right line, except insofar as it is compelled to change that state by forces impressed thereon.

In his 1687 work Philosophiæ Naturalis Principia Mathematica, Newton defined inertia as a property:

DEFINITION III. The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to persevere in its present state, whether it be of rest or of moving uniformly forward in a right line.^[8]