Relativistic Mass:

[beecee]

In my travels I have heard it said that "relativistic mass" is an outdated concept. It always had me wondering about its validity but I was never really sure how to conduct an argument against what I saw as wrong. 

Why did I see it as wrong? Probably two reasons that I know of...[1] As a body gains speed, the harder it gets to get it to continue to accelerate, illustrated by the fact that the energy would need to be infinite to push it beyond "c"...the second being that light/photons have zero rest mass, but yet is able to exert a force due to its momentum.

Am I wrong? or do the points I made mean that relativistic mass is certainly a concept still useful and valid?

[swansont]

Relativistic mass is just a proxy for total energy, so at best it’s redundant.

In a way it’s like the Bohr atom, in that it’s a simple model, but it leads to problems if you try to apply it beyond superficial cases - there are certain equations where you need to use rest mass, so having relativistic mass around only leads to confusion.

[J.C.MacSwell]

1 hour ago, beecee said:

In my travels I have heard it said that "relativistic mass" is an outdated concept. It always had me wondering about its validity but I was never really sure how to conduct an argument against what I saw as wrong. 

Why did I see it as wrong? Probably two reasons that I know of...[1] As a body gains speed, the harder it gets to get it to continue to accelerate, illustrated by the fact that the energy would need to be infinite to push it beyond "c"...the second being that light/photons have zero rest mass, but yet is able to exert a force due to its momentum.

Am I wrong? or do the points I made mean that relativistic mass is certainly a concept still useful and valid?

..actually just to get it to "c"

It can be useful in some respect...it represents the contribution to the mass of a system, by a particle of the system, when measured wrt the frame of the system. I'm not sure why that tends to be confusing but it's use has certainly has fallen out of favour...as Swansont points out it is redundant if you use total energy.

[beecee]

53 minutes ago, swansont said:

Relativistic mass is just a proxy for total energy, so at best it’s redundant.

 

6 minutes ago, J.C.MacSwell said:

..actually just to get it to "c"

.as Swansont points out it is redundant if you use total energy.

Hmmm, OK, I see...I may have to change my ways. Don't get me wrong...I have always viewed it as energy being added to the system so to speak. 

And obviously the following supports both your views....

Thanks for the answers and your efforts in trying to shift an old fart from his rather entrenched position! 

[MigL]

It can be a confusing term.
I still occasionally use it ( old habits can be hard to break ).
Somewhat like the term 'particle' to describe quantum objects, you have to keep in mind when it can be used, and when it can't.

OT:
Sorry about the Australian women's soccer team, mate.
Those girls played their hearts out.
I was saddened to see them eliminated.

[beecee]

1 minute ago, MigL said:

It can be a confusing term.
I still occasionally use it ( old habits can be hard to break ).
Somewhat like the term 'particle' to describe quantum objects, you have to keep in mind when it can be used, and when it can't.

Agreed....

Quote

OT:
Sorry about the Australian women's soccer team, mate.
Those girls played their hearts out.
I was saddened to see them eliminated.

Yeah the Matilda's done well!  Being a Rugby League man though, I'm waiting anxiously for the greatest RL contest to take place tonight...State of Origin between NSW and Q,Land!!

[Strange]

19 minutes ago, MigL said:

It can be a confusing term.

Exactly. For example, it can lead to questions like “would I turn into a black hole if I moved at nearly the speed of light” That us why some people prefer not to use it as a didactic tool. 

It is still a useful concept, once you understand what it means!