First slide
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Question

A particle is projected from horizontal making an angle of 530 with initial velocity of 100 ms-1. The time taken by the particle to make angle 45° from horizontal is 

Moderate
Solution

Component (100 cos 530) 60 ms-1 will remain unchanged.
Velocity will make 450 with horizontal when vertical component also becomes ± 60 ms-1.

Using  v=u+a t   (In vertical direction)

         +60=80+(-10)t1

  t1=2 s-60=80+(-10)t2

  t2=14 s

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