What is the Rocket's Velocity After Emitting Radiation?

[luxiaolei]

Homework Statement

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.

Homework Equations

p^2 = (mc)^2

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.

Help! Thanks in advance!

 

[tiny-tim]

Hi luxiaolei!

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.
…
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.

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

 

[luxiaolei]

Hi luxiaolei!


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

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