Aristotle explained the continued motion of projectiles, after being separated from their projector, as an (itself unexplained) action of the surrounding medium continuing to move the projectile. Aristotle said that all moving objects (on Earth) eventually come to rest unless an external power (force) continued to move them. This misled the philosopher Aristotle to believe that objects would move only as long as force was applied to them. On the surface of the Earth, the inertia property of physical objects is often masked by gravity and the effects of friction and air resistance, both of which tend to decrease the speed of moving objects (commonly to the point of rest). Before the European Renaissance, the prevailing theory of motion in western philosophy was that of Aristotle (335 BCE to 322 BCE). Lienhard points out the Mozi – based on a Chinese text from the Warring States period (475–221 BCE) – as having given the first description of inertia. History and development of the concept Early understanding of inertial motion It is still used today to describe the motion of objects and how they are affected by the applied forces on them. The principle of inertia is one of the fundamental principles in classical physics. 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. In his monumental Philosophiæ Naturalis Principia Mathematica, Isaac Newton defined inertia as a force:ĭEFINITION III.

Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems. An aspect of this property is the tendency of objects to keep moving in a straight line at a constant speed when no forces act upon them. This includes changes to the object's speed or direction of motion. The term inertia may also refer to the resistance of any physical object to a change in its velocity. The term inertia comes from the Latin word iners, meaning idle, sluggish. The modern use follows from some changes in Newton's original mechanics (as stated in the Principia) made by Euler, d'Alembert, and other Cartesians. Other, less forceful terms such as 'to continue' or 'to remain' are commonly found in modern textbooks. The word 'perseveres' is a direct translation from Newton's Latin. Every object perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon. Īfter some other definitions, Newton states in his first law of motion: The term is properly understood as shorthand for 'the principle of inertia' as described by Newton in his first law of motion.

Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change.

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