The total energy of a particle is its rest energy plus the kinetic energy. Its formula is: Et= m^2/sqrt((1-v^2)/c^2)) The rest energy is equal to the product of mass and the square of light velocity: Er=mc^2. When the kinetics energy is twice its rest energy this holds: Et=Er m^2/sqrt((1-v^2)/c^2))=mc^2. sqrt((1-v^2)/c^2))=m/c^2 => v=sqrt(3/2)c
