Re: Constraint and inertia

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

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.

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.

In fact I do not have time to make a clear distinction between the
two attributes. It would be artificial to use the term inertia in this
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?

"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