# A Tough Projectile

A ball is projected with a speed of $$20 \text{ m/s}$$ at an angle of $$30^\circ$$ from a point on the top of a very high tower. The time after which its velocity becomes perpendicular to the velocity of projection (take g = 10 $$m/s^2$$) is

If you have the solution then please post it with an explanation.

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