# Why Is Linear Inertia Only A Property? [closed]

The Inertia of a body is said to be its property or virtue that is directly proportional to its mass. Now if we consider Inertia of rectilinear motion, it depends on the mass as well as the deceleration. I want to know if it is possible to accumulate all such factors and then assign a constant of proportionality so that a formula can be derived that measures Inertia of Linear motion. And if this is possible, on what grounds will the constant be defined? For example: The way we have the coefficient of thermal expansion for different objects, in the same manner, is it possible to have a characteristic constant for a body's Inertia that will turn proportionality into equality?
Note that I am NOT referring to moment of Inertia.

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Are you looking for $p=mv$? This question isn't very clear. – Colin K Jun 2 '12 at 18:01
While google turns up a lot of hits for "linear inertia" this is not a term in widespread use by physicists. The couple of links I followed seem to use it as a synonym for "linear momentum" as @ColinK suggests. – dmckee Jun 2 '12 at 18:07
No, linear momentum clearly refers to Newton's Second Law. What I am curious about is the inability of a body to change its state that is spoken of in the Fist Law. – Graviton Jun 2 '12 at 18:07
You've got it the wrong way -- inertia doesn't depend on the force being applied, but the force needed to produce a desired deceleration is dependent on how much the body opposes such changes. That opposition is called inertia, and is quantified by the mass. Now, how much "opposition" a body puts up is independent of the direction in which you push it. That claim is not easy to experiment -- friction makes life complicated. Think of pushing something suspended in vacuum, something most people have little real experience in dealing with. – Anonymous Jun 2 '12 at 18:29
I can usually work out what a question really means, but I must admit even I'm stumped here. You mention Newton's first law as if it were different from the second law, but Netwon's first law is just a special case of the second law ($F = ma$) where $F$ is zero, so the first and second laws are really the same. – John Rennie Jun 2 '12 at 18:30