1. The problem statement, all variables and given/known data A ball of mass 3m collides head-on with a initially stationary ball of mass m. No kinetic energy is transformed into heat, or sound. In what direction is the mass-3m ball moving after the collision, and how fast is it going compared to its original velocity? 2. Relevant equations p=mv 3. The attempt at a solution I initially set it up as p1i +p2i = p1f +p2f p2i being zero it was discarded from the equation so p1i = p1f + p2f 3mv = 3mv1f + mv2f since m is a unknown constant I can remove it from the equation 3v = 3v1f + v2f unfortunately, this is where I am stuck :( or should I have calculated this via Kinetic energy?