<P> Inertia is the resistance of any physical object to any change in its state of motion . This includes changes to the object's speed, direction, or state of rest . </P> <P> Inertia is also defined as the tendency of objects to keep moving in a straight line at a constant velocity . The principle of inertia is one of the fundamental principles in classical physics that are still used to describe the motion of objects and how they are affected by the applied forces on them . </P> <P> Inertia comes from the Latin word, iners, meaning idle, sluggish . Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems . Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states: </P> <P> 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 preserve its present state, whether it be of rest or of moving uniformly forward in a straight line . </P>

What physical quantity is a measure of the amount of inertia and object has