1. The problem statement, all variables and given/known data By resolving the momentum equations parallel and perpendicular to the incident beam direction, show that α is related to the speed v2 of the recoiling particle by: cos α = m2*v2/(2μ*u1) where μ=m1m2/(m1+m2) 2. Relevant equations See attachment for diagram. m1v1cosθ +m2v2 cosψ = m1u1 ( parallel equation ) m1v1sinθ + m2v2 sinΨ =0 ( perpendicular equation ) 3. The attempt at a solution expressing v1 from the perpendicular equation, then v1 was substituted into the parallel equation, and rearranging for v2 i got cos α = m2v2/m1u1 , which is not the right thing. What am I doing wrong?