Let's consider

$$E\_f^2=(mc^2)^2+(pc)^2$$

where the $mc^2$ is the rest energy due to the rest mass -- in Finnish "lepomassa". 

$$ \sqrt{(mc^2)^2+(pc)^2} - mc^2~=~(\gamma-1)mc^2$$ 

is the kinetic energy due to the movement because of momentum $p=\gamma mv$. 

Now where is potential energy if $E\_f=\gamma mc^2$ is the total energy?