Question

^+--->pi+ u- (^ is exploding and pi and u exists.) m(u)=0 m(pi)=m(^)/6. also Velocity(^)=0 . What's the Velocity of "pi" ?

Answer 1

The momentum of the pi and u must have the same magnitude by conservation of momentum. The relativistic energy of ^ is m^c^2, that of pi and u
sqrt((pc)^2+(mc^2)^2), where m is either the pi or u mass. Set up the conservation of energy and solve for p in terms of everything else.
Finally, v/c = pc/E for the pi.