# Homework Help: Special relativity

1. Nov 15, 2015

### whatisreality

1. The problem statement, all variables and given/known data
A particle with mass M at rest decays into two particles with masses ma=0.15M and mb=0.65M .
i) Discuss the relative directions and magnitudes of the momenta of these two particles in the centre of mass reference frame.

2. Relevant equations

3. The attempt at a solution
What's the centre of mass reference frame?! Isn't there really only one relevant frame here?

As for the relative directions - just in opposite directions, by conservation of momentum? And with the same magnitude of momentum? Although them having the same magnitude of momentum seems weird, because that would be just like Newtonian mechanics, and using that never seems to work anymore ;)

Last edited: Nov 15, 2015
2. Nov 15, 2015

### BvU

Use the definition ! For one particle (your initial situation) it is really easy and quite intuitive !
Yes, yes, and: never say never again! Newton always works in the nonrelativistic limit. It's just that with high velocities you have to use the right relationships for velocity-momentum, kinetic-total energy etc.