Two particles A and B of mass 2kg and 3kg respectively are moving head on. A is moving at 5m/s and B is moving at 4m/s. After the collision, A rebounds at 4m/s. What is the speed of B and what direction is it moving in?

Using conservation of momentum, m1u1 + m2u2 = m1v1 + m2v2, set m1 = 2kg, m2 = 3kg, u1 = 5m/s, u2 = -4m/s and v1 = -4m/s. Substitute these into the equation to get 25 + 3(-4) = 2*(-4) + 3*v2 10 - 12 = -8 +3v2 6 = 3v2 v2 = 2m/s and is moving in the positive direction, away from A.