"A projectile of mass m1 moving with speed v1 in the -x direction strikes a stationary target of mass m2 head-on. The collision is elastic. Use the Momentum Principle and the Energy Principle to determine the final velocities of the projectile and target, making no approximations concerning the masses. " All I've gotten so far is that: P1f + P2f = P1i + P2i but v2i = 0 SO P2i = 0 am I on the right track? I'm pretty much lost at this point.