Inertia and Physics- was "Key Inertia"

[Richard Brekne]

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