# Homework Help: Special Relativity,roket problem

1. Feb 27, 2010

### luxiaolei

1. The problem statement, all variables and given/known data
A rocket propels itself rectilinearly by emitting radiation in the direction opposite to its motion, thereby decreasing its rest mass. What is the velocity of the rocket relative to its initial rest frame after the rocket has radiated off 2/3 of its initial rest mass.

2. Relevant equations
p^2 = (mc)^2

3. The attempt at a solution
I have tried using 4-momentum conservation, and I found the gama of the rocket in final state is 1, hence the velocity is zero! I am sure i must be wrong somewhere.

However, as I read the question again, I found that there is not enough infomation to determine the final state of the rocket, I mean the rate of emitting gas is a parameter of the final velocity of the rocket.

2. Feb 27, 2010

### tiny-tim

Hi luxiaolei!
No, there's no gas … "radiation" in an examination question means light (and you know how fast that goes! )

3. Feb 27, 2010

### luxiaolei

Thanks! tiny-tim!!! So silly I am... I sort it!