What is Inertia

[Sarah Fox]

Hi all,

Sorry for the absence.  I'm back online now, just in time to wish everyone a
Merry Hannukwanzamas.

I agree with Don that we're essentially in agreement about these terms.  I
just thought it would be useful to point out that Christmas affords us a
marvelous opportunity to study the effects of inertia -- that objects at
rest have a tendancy to stay at rest until acted upon by an outside force.
The more massive the object, the greater the inertia, the greater the force
needed to set it into motion -- or halt its motion -- or change its motion.
The night before, the less massive little ones are the first and easiest to
go down, coming to rest in their beds upon merely being reminded that a much
more massive person will soon be visiting.  Very easy.  The bigger, older
folks keep on cranking well into the morning, assembling pieces of plastic
with the aid of incomplete instructions in Chinenglish.  Finally, hopefully,
they come to rest as well, but only after being worn down by the performance
of their annual herculean duty.

About 6:00 in the morning, the little folks bound out of bed and go flying
about the house, seemingly spontaneously.  (They are exhibiting considerable
kinetic energy at this point.)  They very quickly crash through the doors
where the bigger, more massive folks are at rest, and collide with them
repeatedly until they are finally -- slowly but surely -- set into a slow
motion.  At this point, something is needed to accelerate these folks even
more -- coffee -- lots of it.  Finally, with very substantial pushing,
prodding, and caffeination, the bigger, more massive people have achieved
adequate kinetic energy to face the day.

Now, the littler, less massive folks quickly demolish the wrapping paper on
their presents, but their low inertia is such that it takes very little to
slow them down.  As they rip their way through the last of their presents
and play with half of them for a few minutes, they crash, assuming the same
dormant state as they did the previous night.  Meanwhile, the larger, more
massive folks keep plodding along, full of inertia and kinetic energy,
picking up paper, stuffing trash cans, stuffing turkeys (not with paper,
hopefully), baking pies, etc.  Shortly thereafter, all the folks, both more
massive and less massive, stuff themselves with food, thereby gaining
inertia in preparation for the next year.

I hope Santa brings y'all what you wanted.  I see one of us got a table saw!
;-)

Merry Hannukwanzamas, y'all!  Don't eat too many cookies!

Peace,
Sarah




----- Original Message ----- 
From: "Don A. Gilmore" <eromlignod@kc.rr.com>
To: "College and University Technicians" <caut@ptg.org>; "Newtonburg"
<pianotech@ptg.org>
Sent: Wednesday, December 24, 2003 3:25 PM
Subject: Re: What is Inertia


>
> ----- 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
>
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