What is Inertia

[Richard Brekne]

"Don A. Gilmore" wrote:
> 
> There are no units of "inertia"; one  object cannot have more "inertia" than another.  It can have more kinetic
> energy, or momentum, or mass, or velocity, or indeed "moment" of inertia
> than another object since those are measurable, quantifiable properties.


I understand exactly what you are saying, as I understand exactly what
the others are saying. But I have to point out (without taking a
position on the matter myself) that there are three declared definitions
for inertia on pianotech by various folks with some degree of physicis
knowledge. Let me list them.


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.

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 

"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."

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.


grin.... NOW I will state my own position... tentatively...ok ?? :)
Seems to me that Don is correct... except I have a hard time
understanding or accepting that "one  object cannot have more "inertia"
than another". If this is true then either inertia is a constant, or
inertia is just plain undefined... as in divideing by zero more or less.
So I lean towards Sarah and Mark. But I want to see you 4 hashing this
out so we can past the problem.... as clearly any discussion about
action mechanics on this list is going to be rather meaningless unless
we can agree on what terms like inertia mean.

Cheers
RicB