# Exploding question

asdf1
In this question, "A stationary body explodes into two fragments each of mass 1.0 kg that move apart at speeds of 0.6c relative to the original body. Find the mass of the original body."

My first thought is that you need to use the law of conservation of momentum.

However, how do you take relativity into account since the pieces move near the speed of light?

El Hombre Invisible
That question may be misleading. If, by "mass 1.0 kg" it refers to rest mass, aka invariant mass, then the original mass was simply 2.0 kg. If it meant relativistic mass, then you need to derive the rest mass from that (i.e. m(root(1-v^2/c^2))).

asdf1
@@a
jeepers! i didn't think of that...
getting more and more complexed~

El Hombre Invisible
Generally, 'mass' is taken to mean invariant mass (i.e. the mass at rest). If someone wanted to specify relativistic mass, they would say 'relativistic mass' or 'relative mass'. I think the whole idea of relativistic mass of an already massive object in SR is somewhat misleading and pointless. It is much more relevant to the relative mass of energy, such as binding energy, that does not have a rest mass.

asdf1
wow~
thanks! :)