http://img530.imageshack.us/img530/7629/pnxl1.gif [Broken] A particle p traveling with a speed of vpi = 3 m/s hits and scatters elastically from another particle N, initially at rest. Particle p is deflected through 90°, leaving with a speed of vpf = 2.7 m/s, and a mass mp = 2 kg. a) What angle (in degrees) does the recoiling N make to the incident-p direction? b) What is the magnitude of the momentum of the recoiling N? c) What is the change in the energy of the p? d) What is the speed of the recoiling N? e) What is the mass of the N? Relevant equations: Conservation of momentum and energy I think i have to break up the momentum into x and y directions. I did an equation like this... x-direction: (2Kg)(3m/s) + (mass-N)(0m/s) = (2kg)(0m/s) + (mass-N)(Vfinal-N)*cos(theta) theta being the angle were looking for. But I'm getting stuck because i don't know the mass or the final velocity of particle N. There has to be another way to do this i just can't think of it.