Re: Dumb Relativity Question

From: The Twisted Dreams of Fascinet (fascinet_at_yahoo.com)
Date: 02/11/05 

Date: 11 Feb 2005 13:37:29 -0800

Franz Heymann wrote:
> "The Drums of Fascinet" <fascinet@yahoo.com> wrote in message
> news:1108077638.063340.254900@o13g2000cwo.googlegroups.com...
> > So, when you have two objects inelastically collide at relativistic
> > speeds such that they squish together and have no velocity in what
> was
> > the CM frame, their new mass is greater than the sum of the
original
> > objects' masses in their respective frames.
>
> Certainly.
> >
> > Easy to prove, no big deal.
>
> > So, if two objects are placed such that they'll accelerate from
rest
> to
> > the speeds in the first paragraph, after the collision, the
> resultant
> > object will have a larger mass than the sum of two masses when they
> > were at rest, for all the same reasons.
>
> Yes, of course.
>
> >
> > If someone in what will be the CM frame at the time of collision
> > measures the the pull of the system a long way out, what will he
> record
> > as the original mass of the system?
>
> The mass of the system is an invariant.
>

Is that to say that the attractive agent between the two objects has a
mass of the difference in the rest mass of the original two objects and
that of the system as a whole?

That is, and I doubt this uses the terms correctly, if the rest mass of
each of the original objects is M and the rest mass of the final object
is (5/2)M, then are we committed to saying that the mass of the
potential energy is (1/2)M?

-F

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