A particle is projected upwards with a velocity of 100 m/sec at an angle of 60° with the vertical. Find the time when the particle will move perpendicular to its initial direction, taking g = 10 m/sec2:

Method I: u→=100cos30°i^+100sin30°j^=503i^+50j^v=u→+g→t=503i^+50j^+−10jt=503i^+50−10tj^if u→⊥v→, then u→.v→=0 so5032+5050−10t=0⇒t=20 second Method II: By v=v+at=u+gtFrom right angled triangleugt=cos60°⇒t=20s