Q. A stone is thrown from the top of a tower of height 50m with a velocity of 30*ms*^{-1} at an angle of 30° above the horizontal. Find the time during which the stone will be in air

(a) 2 sec

(b) 3sec

(d) 4sec

(d) 5 sec

Ans: (a)

Sol:

$ \displaystyle -h = usin\theta t – \frac{1}{2}g t^2 $

$ \displaystyle -50 = 30(\frac{1}{2})t – \frac{1}{2}\times 10 t^2 $

-50 = 15t -5 t^{2}

t = 5 sec