Re: Constraint and inertia

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

On Thu, 11 Nov 1999, Ludwik Kowalski wrote:

> The term mass is often abused by us. A phrase "the speed of
> mass A" should be replaced by the "speed of an object A", as
> emphasized by others last week. Saying the "speed of mass"
> is like saying the "speed of color", or the "speed of electrical
> resistance", etc. Mass is an attribute of an object, not the
> object itself.

...and f(x) is not the function, but the image of the argument x through
the function f. However, it is generaly accepted to identify the function
by its image.
 I don't see any abuse on what I said. BTW, the only thing we can detect
are attributes, and I'm not willing to start a philosophycal discussion on
ontology :)


> Once we correct this we have to decide about names of attributes.
> I do not like "inertia being a quantity expressed in kg". The terms
> "mass" and "gravitational mass" are probably better than the terms
> inertia and mass. Why? Because of the language. In common
> terms the mass is the amount of substance which can not be changed
> without taking something away, or adding. In an introductory course
> I do not reject this naive picture. The definition of m in terms of F/a
> is too complex to begin with.

 Ok, for the intro course, but once you'd taught N2L, would it still be
too complex to define it in terms of F/a? I don't feel so...

> teaching environment. So I prefer to think that inertia is a tendency
> an object always has to preserve velocity. I do not treat is as a
> physical
> quantity in kg. Is it a sin?

 Shouldn't we talk only about measurable quantities? You can measure that
tendency in terms of F/a, which has units of Kg.

 Any way, I don't see any problem in mixing both ideas, inertia and amount
of substance when talking about F/m, or may be there is some problem...?

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




> "Miguel A. Santos" wrote:
>
> > On Thu, 11 Nov 1999, Ludwik Kowalski wrote:
> >
> > > Some people say inertia instead of mass. Why do we need
> > > another term for mass? Inertia is not a physical quantity.
> > > Newton's first law is called the law of inertia, the law of
> > > tendency to preserve velocity.
> >  But after F=ma, it is easier to change the velocity of a lower
> > mass m. And reversly, the greater the mass the greater its "tendency
> > to preserve velocity". Mass gives us an idea of the inertia of a body.
> >
> >  BTW, how would you explain what is mass (the one of F=ma) without
> > refering to inertia?
> >
> >  Miguel A. Santos
> >  msantos@etse.urv.es