1. The problem statement, all variables and given/known data A sphere of mass m1 travelling at velocity u1 collides elastically with a stationary sphere of m2. The particles move away with v1 and v2 respectively. I'm at part 4, of 5. It asks us to resolve the conservation of momentum equations normal and tangential to the post-collision velocity of m2. I obtained these equations using the diagram that's provided. The part I'm stuck on is where they ask us to obtain an expression for tanB involving only the masses and angle A. A = angle of incidence B = angle of reflection The 5th part asks "Hence when does the angle of incidence equal angle of reflection?". So I know the resultant equation should be something along the lines of: tanB = ((m1 + m2)/m2)*tanA as m1 should be << m2 2. Relevant equations m1u1cosA = -m1v1cosB + m2v2 m1u1sinA = -m1v1sinB m1u12 = m1v12 + m2v22 3. The attempt at a solution I found terms for v1 and equated them eventually getting: tanB = ((m1u1cosA)/(m1u1cosA - m2v2))*tanA But now I'm stuck. Any pointers?