Re: [Phys-l] Invariant mass and relativist mass...

[John Denker <jsd@av8n.com>]

On 02/24/2008 02:15 PM, Jacques Rutschmann wrote in part:
.....

> So during my particle physics years, I decided that the teaching of the
> “relativist mass” concept was (at best) confusing and unhelpful.

Yes, that's the modern view.  For details see
    Gary Oas, “On the abuse and use of relativistic mass”
    http://arxiv.org/PS_cache/physics/pdf/0504/0504110v2.pdf

> But now, many years later, I am not sure about this. Let's say I wanted
> to compute the gravitational pull of a car (coasting by my lab) on a
> very sensitive pendulum (the pendulum does not swing: I apply a small
> force on it to keep it steady). The rest mass of the car is know.

OK.

> I am tempted to use the car “relativistic mass” in my computation 

Resist that temptation.

> (This
> is very natural: I believe that most non GR versed people would do the
> same).

But GR-versed people would not.

We agree that mass is "the" source term for the gravitational field in 
Newtonian gravitation.  In contrast, in GR, the source term is quite a
bit more complicated.  It's not even a scalar.  Using some sort of 
"relativistic mass" is not even approximately correct.  

You're not going to learn the right answer via email.  GR is complicated.

If you want a simple Gedankenexperiment to convince you that simple
guessing cannot possibly give the right answer, take a look at
  http://www.av8n.com/physics/gravity-source.htm