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## Inertia – Wikipedia

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.

In Newton’s time all physical objects were considered to be composed of matter. The knowledge of field as a physical substance came later. Therefore, the definition of inertia may be broadened as follows: “Inertia is the resistance of any physical substance to any change in its state of motion”.

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.

If an object is not accelerating, it is not really moving relative to itself. Its constant velocity is fictitious because it is determined by the inertial frame of reference, which can be selected arbitrarily. So the object is pretty much where it is. The inertia resists any effort to move the physical object relative to itself.

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:

“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.”

There is no difference between the state of rest and the state of moving uniformly except for a different inertial frame of reference in use.

In common usage, the term “inertia” may refer to an object’s “amount of resistance to change in velocity” (which is quantified by its mass), or sometimes to its momentum, depending on the context. The term “inertia” is more properly understood as shorthand for “the principle of inertia” as described by Newton in his First Law of Motion: an object not subject to any net external force moves at a constant velocity. Thus, an object will continue moving at its current velocity until some force causes its speed or direction to change.

Inertia is the property of substance of which physical objects are made. Since Newton’s time, field has been discovered as more basic physical substance. Therefore, the property of inertia applies equally to the field. It is the frequency of field that represents its “state of motion”.

On the surface of the Earth, inertia is often masked by the effects of friction and air resistance, both of which tend to decrease the speed of moving objects (commonly to the point of rest), and gravity. This misled the philosopher Aristotle to believe that objects would move only as long as force was applied to them:

“…it [body] stops when the force which is pushing the travelling object has no longer power to push it along…”

The motion worth considering with respect to inertia is acceleration and not the uniform motion.

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