🚀 go-pugleaf

matt@physics.berkeley.edu (Matt Austern) writes:

>In article <1992Jul22.193837.18095@sfu.ca> palmer@sfu.ca (Leigh Palmer) writes:

>> "Mass" as you use it
>> is equal to neither inertial nor gravitational mass as those terms apply to
>> any system more complicated than a particle at rest!

>There is, however, *nothing* which is equal to either inertial or
>gravitational mass for a relativistic body.

>That is: I define the inertial mass of a body as the "m" appearing in
>the equation
>		F = m a,					(1)
[...]
>I believe that these definitions are customary; I don't know what
>other meaning could be attached to the phrases "inertial mass" or
>"gravitational mass."

Wellll, if you define F as dp/dt instead, and then define inertial
mass by the formula p = mv, then the total energy
(or "relativistic mass") *does* become exactly the "inertial mass."
But I still don't think this should be used in physics classes.

As for gravitational mass, I agree with you there: there really
isn't any such thing in the context of GR, unless you want to
define it trivially as the "inertial mass" defined above because
acceleration is independent of mass-- but that's hardly the
best way to look at it, though it's the language that often
gets used.

-- 
Matt McIrvin   mcirvin@husc.harvard.edu   Long live short .sigs