1. The problem statement, all variables and given/known data Two identical particles of mass m travel towards each other at speed v; they combine and form a single new particle. By employing conservation of momentum and conservation of energy, what is the mass of this new particle in 2. Relevant equations Relativistic momentum and total energy , possibly lorentz velocity transformation 3. The attempt at a solution I am completely lost to where to start with this, because of two reasons. I can see if I need to use a frame of reference, I not sure if the v<<c and do I take the velocity as a vector? I know the momentum and total energy are consevered in relativity but in the question I see that if the final momentum is equal to the sum of the initial surely this 0 because they are moving at the same speed and they are the same mass, so how can they even move at a speed after the collision. My lecture has not explained this very well in his notes and I am very lost could someone please help me. Also tell me what the latex tags are for the physics forum. Big thanks in advance.