Re: Inertia or the "amount of substance"

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

>  .... 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.

Good comment. To refresh my memory I looked into Special
Relativity. It begins with frames of reference, length contraction,
the twins paradox, etc. This is kinematics, right? At some point
a distinction between m_zero and m appears, both are inertial
masses. The m_zero is the mass of an object at rest with respect
to my laboratory frame, the m is the mass of the same object
when it has a speed v with respect to me. The formula which
relates the two masses is derived by considering an elastic
collision of two identical particles and by requesting that the
law of conservation of linear momentum should be valid in
all frames of reference. No gravity is involved.

Then the work-energy relation is used to show that the KE
of the object is given by KE=m*c^2-m_zero*c^2. In the
formal derivation the F=m*a is replaced by F=dp/dt. Again,
no gravity is involved.

After that point "a declaration" is made that the second term is
the "total energy" of an object. Gravitational potential energy
is not analyzed anywhere, the identity of two masses (inertial
and gravitational) is silently assumed. Perhaps those who
teach special relativity will correct me. My understanding is
that the inertial mass is the one which appears in 0.5*m*v^2
while the gravitational mass is the one that appears in m*g*h.
Ludwik Kowalski
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