A particle is projected with a certain velocity at an angle α above the horizontal from the foot of an inclined plane of inclination 30°. If the particle strikes the plane normally, thenα is equal to

tAB= time of flight of projectile =2usinα-30°gcos30° Now component of velocity along the plane becomes zero at point B.  0=ucosα-30°-gsin30°×T ucosα-30°=gsin30°×2usinα-30°gcos30° tanα-30°=cot30°2=32 α=30°+tan-132