Special Relativity, energy and momentum

[courtrl]

1. The problem statement, all variables and given/known data

In reference frame S', two protons, each moving at .5c, approach each other head on. A) Calculate the total kinetic energy of the two protons in frame S'. B) Calculate the total kinetic energy of the protons as seen en reference frame S, which is moving with one of the protons.

2. Relevant equations

K=(mc^2/sqrt(1-(v/c)^2))-mc^2

3. The attempt at a solution
I have no attempt. I have no idea where to start.

[nickjer]

Well I will assume you know the answer to A, since you wrote out the equation. And in B, you know the velocity of one of the protons is 0. To get the velocity of the other proton, instead of summing the velocities classically which would get you the 2nd proton moving at the speed of light, c (which is impossible). You would add the velocities relativistically:

http://en.wikipedia.org/wiki/Velocity-addition_formula

[vela]

You could also calculate the energy and momentum of the protons in the S' frame and then use a Lorentz transformation to find their values in the S frame.