If at point of projection, the velocity of a particle is u and is directed at an angle alpha to the horizontal, then show thatit will be moving at right angles to its initial direction after a time ((u cosec alpha))/g

If at point of projection, the velocity of a particle is u and is dire

1.	If at point of projection, the velocity of a particle is u and is directed at an angle alpha to the horizontal, then show thatit will be moving at right angles to its initial direction after a time ((u cosec alpha))/g
Answer» Solution :Let t be the time after which velocity becomes perpendicular to its initial direction. As u and v are perpendicular the angle between v and vertical will be `alpha` Intitial velocity `u=(u cos alpha hati+u sin alpha hatj)` After t SEC velocity `v={u cos alpha hati +(usin alpha-"gt")hatj}` `:.` These are perpendicular their dot PRODUCT is ZERO `:.(ucos alpha i+usin j).{ucos alphai+(usin alpha-"gt")j}=0` and `t=(u cosec alpha)/G`