A particle of mass m moving with velocity 1 m/s collides perfectly elastically with another stationary particle of mass 2m. If the incident particle is deflected by 90°, the heavy mass will make an angle θ with the initial direction of m equal to

From conservation of momentum along incident directionm ×1 = 2 mv2Cosθ                           iFrom conservation of momentum along perpendicular direction,m ×0 + 2 m × 0 = mv1− 2 mv2sinθ⇒v1= 2v2sinθ                        iiFrom energy conservation, 12m×(1)2=12mv12+122mv22Solving we get θ = 30°.