As measured in an inertial coordinate system S, a proton initially traveling in the x direction at speed 0.9c will collide with another proton initially traveling in the opposite direction at the same speed.
a) what are the initial velocities of the protons measured in an inertial coordinate system S' in standard orientation and moving as speed V=0.9c relative to S?
b) what are the energy and momentum of the 2-proton system measured in S'?