1. The problem statement, all variables and given/known data A photon of a given wavelength "lambda" collides with a hydrogen molecule that's initially at rest (photon is absorbed by H2). Assuming absorption event conserves momentum, find velocity and translational energy of H2 after the event. 2. Relevant equations momentum of photon = h/wavelength momentum of object=mv kinetic energy=1/2mv^2 3. The attempt at a solution For the velocity, since momentum is conserved, and H2 is initially at rest: h/wavelength=mv where left side of equation is for photon and right side of equation is for H2. So I plug in values to find "v". But I'm not sure of what to put in for "m". Is it just mass of two hydrogen atoms (which is really the mass of 2 protons) or is it the reduced mass of two hydrogen atoms or protons? (In general, I'm confused as to when to use sum of masses as opposed to reduced mass) And then for the kinetic energy, I guess I would just use 1/2mv^2... where "v" is velocity I find in the first part and "m" is... again, I'm not sure what "m" is.