Inertia and Physics- was "Key Inertia"

[Don A. Gilmore]

Man, Ric, you are just beating this to death.

I have been stating the way inertia should be thought of based on years of
its use in both elementary and advanced concepts like dynamics, kinematics,
vibration, thermofluid mechanics and thier application to gear trains,
linkages, cams, hydraulics, mass transfer and other systems.  As it applies
to the ordinary, beginning Newtonian physics maybe we just ought to give it
a rest.

As long as everyone understands the difference between rotational effects
and linear effects and that kinetic energy applies to moving objects and
mass relates to forces and accelerations regardless of velocity (and I think
nearly everyone does understand this now), I don't think there should be any
problems.  I suppose you can use the word "inertia" however you want as long
as you think of matter and energy in the proper perspective and use the
appropriate formulas for your calculations.  Arguing about the clinical
definition of a word gets us nowhere.

If you look back at what people, including those then accepted as
authorities, were saying just a week ago, you'll see that things were
effectively chaos.  People were making statements, with great hubris, that
made no sense whatever.  Now I think almost everyone has a good grasp on the
situation and we're all talking about the same things.

Don A. Gilmore
Mechanical Engineer
Kansas City

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: Friday, December 26, 2003 7:36 AM
Subject: Re: Inertia and Physics- was "Key Inertia"


>
>
> Paul Chick wrote:
> >
> > Richard,
> >
> ..............
>
> > Force is directly related to mass, so if there is an increase in mass,
there
> > is an increase in force.  If there is more mass, there is more inertia.
> > Agreed?  Example:  if you increase the amount of mass of an object, it
will
> > take more force to get the object to move.  A heavier hammer requires
more
> > force (pushing down on the key) to make it move.  It is also true that
the
> > heavier hammer has more inertia.  For a rotating object instead of
force,
> > torque is used.
>
> Agreed, of course :)
>
> >
> > > Calin Thomasen said today
> > > "Any object has a quantifiable resistance, impedance if you prefer, to
a
> > > change in its velocity to any other given velocity, and that
resistance
> > > is reflected clearly and precisely in the amount of force required to
> > > achieve that acceleration."
> >
> > If resistance or impedance are used to define inertia, fine.  Then
according
> > to F = ma if you increase the mass, force will change.  One can also
argue
> > that according to the equation if you increase acceleration you can
increase
> > force, thus, because acceleration is a change in velocity, a change in
> > velocity also affects force.
>
> Up untill a few days ago this had always been how I'd percieved inertia
> as well. Statements from folks like Jim Ellis seemed to support that,
> and he was by no means alone.
>
> > Well, if this is true, what happens when an
> > object is at rest.  Acceleration and velocity are zero.  Does the object
not
> > have inertia?
>
> Hmm... I see your point, but isnt that rather just one way of
> interpreting this zero product of zero acceleration and velocity ?
> Wouldnt it be just as valid to interpret this as << Resistance to zero
> force is of course also zero >> ?? That would certainly make sense
> (intuitively) and be consistant with the idea that inertia proportional
> to mass and acceleration.
>
> > Looking at the definition of inertia as the "tendancy" of an
> > object not to change its motion (this case zero motion), which in
physics is
> > the accepted definition, an object at rest has inertia.
>
> Hmmm.... again I see the difficulty. We speak of energy as potential or
> kinetic, but we dont think of inertia except in general terms. Still, if
> inertia is on the one hand defined as a "resistance to a change in
> velocity", and at the same time there is nothing trying to change
> velocity.... then whats to resist ??  How can anything resist nothing
> ... to put a point on it ? Ok... physics says an object at rest has
> inertia... but this inertia is really meaningless until some force is
> applied to that object.
>
> So how does a physics teacher explain this to a student who wonders thus
> ?
>
> > I always ask my
> > students "Why does a magician use fine china for the magic trick of
pulling
> > a table cloth out from under the table setting".  The answer is because
fine
> > china has more mass (it is heavier) than regular, cheaper, everyday
dishes.
> > The magician will be more successfull because the non-moving fine china
has
> > more inertia (tendancy not to move) than does cheaper everyday dishes.
> > Because non-moving objects still have inertia, it is accepted in physics
> > that velocity and acceleration do not effect inertia nor are they
affected
> > by inertia.  Because force is directly related to mass, it is
misconceived
> > that in a round-about way acceleration (and velocity) are affected and
they
> > have an effect on inertia because force effects acceleration.  It's not
a
> > two-way street.  An increase in mass -> an increase in inertia.  Not: an
> > increase in inertia -> an increase in mass.  An increase in mass ->
> > (requires) an increase in force.  Not: an increase in force -> an
increase
> > in mass.
>
> > These are reversible: an increase in force <-> an increase in
> > acceleration.
>
> Hmmmm....  if F = ma... and m is not held as a constant... then the
> above just cant be correct.  10 = 5 x 2 ..... and 20 = 10 x 2....
>
>
> > A change in acceleration <-> a change in velocity.
> >
>
> > Paul Chick.
> >
>
> Still... I see your points, and tho I have commented, those comments are
> not meant as me taking any particular position. In the end all that is
> important here is that we all use the term inertia in the same way, and
> that this is compatible with the correct physics definiton.
>
> So, if both Don and Jim, come right out and agree with Sarahs following
> statement and both are in agreement that inertia has nothing to do with
> velocity or acceleration... then we are are all on the same page.
>
> "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."
>
> Cheers, and thanks for your posts Paul.
>
> RicB
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