Q) A projectile is thrown horizontally with a velocity 30m/s . After what time will it be moving at an angle of 30 degree with the horizontal?

A)
Since horizontal velocity remains constant, the body acquires some downward vertical velocity due to gravity which causes resultant velocity to be inclined 30 degrees with horizontal.

Tan30=v/30
v=10.sqrt(3) m/s
now this v=at
v=gt
t= 10.sqrt(3) /10 = sqrt(3) secs

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