What is Inertia

[Don A. Gilmore]

Gentlemen:

I think I can straighten out the concept of inertia with a single statement:

***An object exhibits the effects of inertia even if it is standing
still.***

Inertia is just the stubborn tendency of matter to resist change (kind of
like me).  Matter resists a change in its velocity (acceleration).  That
change can be from sitting still to 10 m/s, or it could be changing it from
100 m/s to 110 m/s.  It resists in the form of force (or torque, for a
rotating object).

F = m * a, for linear motion in a straight line
T = J * [alpha], for rotary motion about an axis

Where F is force (newtons, or lbs), m is mass (kg, or slugs), a is
acceleration (m/s^2, or ft/s^2), T is torque (N-m, or lb-ft), J is moment of
inertia (kg-m^2, or slug-ft^2) and [alpha] is angular acceleration in
radians per second squared.  Note that velocity is not involved.  It takes
the same amount of force to accelerate an object from rest as it does if
it's travelling at 1000 mph.

If you're thinking of an already-moving object as possessing the potential
ability to do work or exert force when it is acted upon (like striking
another object) you are thinking about kinetic energy:

KE = 1/2 * m * v^2, for linear motion in a straight line, or
KE = 1/2 * J * w^2, for rotary motion about an axis.

Where m is mass (kg, or slugs), v is velocity (m/s, or ft/s), J is moment of
inertia (kg-m^2, or slug-ft^2) and w is angular velocity in radians/sec.
The answer will be in energy units of either joules (metric), or ft-lbs
(English).

As you can see both properties can vary depending on mass, but only the
latter takes velocity into account.  Of a fast-moving object you could say
that it "has a lot of kinetic energy", but you would *not* say that it "has
a lot of inertia".

Moving objects possess a potentially usable energy.  Inertia is
stubbornness.

Don A. Gilmore
Mechanical Engineer
Kansas City

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "College and University Technicians" <caut@ptg.org>
Sent: Tuesday, December 23, 2003 3:16 AM
Subject: What is Inertia


> Ok... getting closer to a common definition... of inertia / moment of
> inertia.  But it seems like very many of us are working with an inertia
> concept that is tied to velocity in some sense, and Jims post below
> doesnt seem to be an exception really.
>
> Perhaps this idea comes from the old "snowball rolling down hill"
> analogy... and we think more along the lines the thing picks up speed,
> and not so much that it gets bigger..
>
> In anycase... Jim... what exactly do you mean by
>
> "the moment of inertia is proportional to the square of the radius about
> the center of rotation, or the pivot, but that relates back to the
> change in velocity, whether it be acceleration or deceleration."
>
> How exactly does it relate back to a change in velocity ?
>
> Cheers
> RicB
>
> James Ellis wrote:
> >
> > Don A. Gilmore, mechanical engineer, Kansas City, is exactly right.  I
used
> > the wrong words.  I should have said "property" instead of "quantity",
and
> > I should have said "acceleration" instead of "velocity".  Also,   So my
first statement was not
> > so far off after all.  Other than that, I'm saying the same thing Don
is,
> > and I'm also sticking to my guns about the point I'm trying to make.
>
>
> Cheers
> RicB
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