Q. A ball is dropped from the top of a building. The ball takes 0.5s to fall past the 3m length of a window at certain distance from the top of the building. Speed of the ball as it crosses the top edge of the window is (g = 10 m/s^{2})

(a) 3.5 ms^{-1}

(b) 8.5 ms^{-1}

(c) 5 ms^{-1}

(d) 12 ms^{-1}

Ans: (a)

Sol:

Applying $\displaystyle S = ut + \frac{1}{2}gt^2$

$\displaystyle 3 = u\times 0.5 + \frac{1}{2}\times 10 (0.5)^2$

$\displaystyle 3 = u/2 + 5/4 $

u = 3.5 m/s