What is Inertia

[Don A. Gilmore]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "College and University Technicians" <caut@ptg.org>; "Newtonburg"
<pianotech@ptg.org>
Sent: Wednesday, December 24, 2003 5:22 AM
Subject: Re: What is Inertia


> 1. Don Gilmore... inertia is a concept, not a quantity, has nothing to
> do with size, mass, velocity or anything else. Is simply the fact that
> objects with mass tend to resist any change in velocity. No object
> regardless of mass has any more inertia then any other mass.
>
> 2. Sarah and Mark.... inertia is very much like Don describes, yet
> inertia is mass related... a larger mass will definatly have more
> inertia then a smaller mass.

This is about as close as we're going to get here.  I'm not going to
nitpick.

> 3. Jim Ellis.  inertia is clearly mass related its very hard to read his
> definition without concluding he means that inertia is related to
> acceleration and /or velocity... That  relation to acceleration seems a
> bit unclear... but as I read through his posts I get that he first
> said... Inertia = mass x velocity-squared, then after some debate
> changed this to Inertia = mass x acceleration-squared. His last post
> seemed to draw this up a bit differently

When Jim says "mass x velocity-squared" he is probably referring to kinetic
energy (which is actually 1/2 that much), which would be in energy units of
joules, or foot-pounds.  The reference to acceleration-squared is probably
just a typo.  Acceleration squared doesn't apply to anything in physics.

> "Inertia is a minifestation, a property, an effect, of acceleration and
> deceleration.  It's proportional to the square of the change in speed,
> or velocity."

I'm not sure who that quote came from but  it's *way* out there.  This is
nonsense pulled out of the air.

> What I'd like to see at this point is that since Don, Sarah, Mark, and
> Jim all are people we all rely on for physics insights, and because they
> all present clearly different definitions of this concept,,, that these
> four all bang this one through until they arrive at a common definiton
> for us.

I think we're pretty much there.  Just remember:

1.  Moment of inertia is like "rotational mass"
2.  Objects in motion have kinetic energy.

I have to use this stuff every day.  This is freshman-level physics.
Believe me, it gets a lot more complicated than this.

Don A. Gilmore
Mechanical Engineer
Kansas City