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**1. Homework Statement**

A rocket uses a photon radiation engine. Knowing that from a reference R the initial and final rest masses of the rocket are Mi and Mf show that

Mi / Mf = sqrt [ ( c + v ) / ( c - v ) ]

and that the rocket was initially resting on R

**2. Homework Equations**

Total Energy = Mo.c^2 + T (kinetic)

T= Mo.(c^2).{ [1 - (v^2 / c^2)]^(-1) -1 }

**3. The Attempt at a Solution**

I calculated the total energy on the instants i and f, like this

Ei = Mi.c^2

Ef = Mf.c^2 + Tf

and since there's conservation of the total energy Ei = Ef

in the end i got

Mi / Mf = sqrt [ ( c^2 ) / (c^2 - v^2) ]

so... i must have assumed something wrong ;/

any tips?