What is the relative velocity of the two reference frames?

[nellywood]

Homework Statement

A particle as observed in a certain reference frame has energy 5GeV and momentum 3GeV. what is the energy in a frame in which its momentum is equal to 4GeV/c?
what is its rest mass?
what is the relative velocity of the two reference frames?

Homework Equations

The Attempt at a Solution

v=(pc^2)/e
v=0.6c which is the relative velocity of the 2 frames

at v=0
e=mc^2
therefore m=e/m^2=8.88*10^-27 which is the rest mass of the particle

 

[grey_earl]

I would use the relativistic momentum conservation law
$$E^2/c^2 + \vec{p}^2 = m^2 c^2$$
in both frames. Afterwards you could use your formula to calculate the relative velocity of those two frames.