Re: Inertia or the "amount of substance"

["Miguel A. Santos" <msantos@ETSE.URV.ES>]

On Sat, 13 Nov 1999, Ludwik Kowalski wrote:

> My comments were about the way of introducing a concept,
> not about ways of refining it in subsequent courses. You are
> correct, Miguel, initially introduced concepts must be refined,
> as needed. Teaching would be impossible without this. A
> misconception, I think, exists when a statement is nearly
> always wrong, not when situations in which it is wrong are
> uncommon.

 I realized your point and thus I agreed on using the idea of "amount of
substance..." in an intro course (12-15yrs old kids). It's just that i
felt it necessary to have clear to what extend an idea can be stressed
while using it.
 Also, as Kirkpatrick says, it may be more a misleading idea than a
misconception.
 However, I'd say that still in those intro courses, the idea of mass as
inertia wouldn't be too bulky or awkward for them. They already have the
feeling of how difficult is trying to push different bodies... Well, I
recognize I'm not sure of this to be an appropiate orientation for those
levels.

> A discussion on how to introduce "getting fatter and fatter",
> when v-->c, would be interesting. Is it an illusion or is it real?
> And what happens to the mass? Is this the area in which a
> distinction between the "two masses" becomes essential?
> Ludwik Kowalski


 I'm not sure that I understand your last question completely. But, let me
add a comment to yours when you stated that the m of "E=mc^2" is only of
kinetic origin: What about the rest mass? When you move relative to two
masses, which are at rest one with respect to the other, their masses
include kinetic as well as other energies: gravitational, chemical,
thermal energy,...Hence, not kinetic origin alone.

 Regards,
        Miguel A. Santos
        msantos@etse.urv.es


> "Miguel A. Santos" wrote:
>
> > ... I'd say that the conception of mass "as the amount of
> > substance which can not be changed without taken somehting
> > away, or adding" may be a misconception*. Namely, as I move
> > faster and faster, relative to someone else, I see her/him getting
> > fatter and fatter, and she/he is not eating that much. It is the
> > inertia which is increasing. .....