A particle is projected with a certain velocity at an angle α above the horizontal from the foot of an inclined plane of inclination 300. If the particle strikes the plane normally then α is equal to:

tAB = time of flight of projectile         = 2u sin(α-300)g cos 300Now component of velocity along the plane becomes zero at point B.θ = u cos(α-300)-g sin 300×Tor   u cos(α-300)   = g sin 300× 2u sin(α-300)g cos 300or tan(α-300) = cot 3002 = 32   α = 300+tan-1(32)