# Homework Help: Photonic rockets

1. Oct 25, 2007

### liskawc

1. The problem statement, all variables and given/known data
one is powering a photonic rocket with photons. what is the speed it reaches when its mass is half its starting mass.
and per say you would start stoping then ... how would that look like (equations and sentences)

2. Relevant equations
invariance of the 4D vector of momentum (W/c)^2 - p^2=invariant

3. The attempt at a solution
i know the problem is solvable using the law that energy is constant and that momentum is comstant but i was wondering how would one do this using the above equation ... i d prolly need to consider the momentum of the light the rocket emitted, yes?

2. Oct 25, 2007

### Dick

The four momentum of the total system is (m,0). You aren't going to get much out of that. You need to combine three vectors to get the result. And I gather you already know how to do that.

3. Oct 26, 2007

### liskawc

aye ...
well i just wondered of alternative methods :D