## A Conversation for Inertial Frames

### Inertial frames and special relativity

Math - Playing Devil's Advocate Started conversation Aug 2, 2003

As far as I can recall its from special relativity we get the idea that mass increases with velocity, if I'm incorrect in that then I guess its inertial frames and somewhat else...

If two objects A and B for simplicty, are closing at a combined speed of 0.8c, each has a mass of 1kg (again to keep it simple)

Let us consider this from the point of view that A is stationary, and that B is moving towards A with a speed of 0.8c, this then increases the mass of B.

Let us compare this with the view that B is stationary, we should get the same results, but however now we calculate the mass of B to remain at 1kg and the mass of A to change.

A third simple intertial frame occurs to me, and may be worth comparison, the concept that both are moving with equal velocity. So now both A and B are moving at 0.4c and thus neither has a mass of 1kg. I find this interesting because if I recall correctly the formula for the change in mass is nonlinear, so in this view we have changed the momentum of this closed system, merely by changing perspective.

This leads me to conclude that one of these theories is incorrect, unless of course my application of the theory is in some manor incorrect.

Math

### Inertial frames and special relativity

Noggin the Nog Posted Aug 2, 2003

Isn't change of mass with change of velocity a feature of general relativity, which includes gravitational fields, rather than special relativity?

Noggin

### Inertial frames and special relativity

Math - Playing Devil's Advocate Posted Aug 2, 2003

It may well be, it's almost 7 years since I studied physics properly, but is my application of the theory correct, or does it have some special clause that tidies up the problem with inertial frames ?

### Inertial frames and special relativity

Noggin the Nog Posted Aug 2, 2003

As far as SR is concerned, objects only move relative to each other - so I suppose it depends on who is doing the measuring. But you'd be better off asking a physicist.

Noggin

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