A stone is dropped from a height of 20 m.

(i) How long will it take to reach the ground?


(ii) What will be its speed when it hits the ground? (g = 10 m/s2)

Height, s=20m

Initial velocity, u=0


Acceleration due to gravity, g=10m/s2


Final velocity, v=?


Time taken, t=?


(i) Using relation,


s = ut + gt2


20 = 0 * t + * 10 * t2


20 = 0 + 5t2


t2 = = 4


t = 2s


(ii) For a freely falling body:


v2 = u2 + 2gh


= (0)2 + 2 * (10) * (20)


= 400


v = 20m/s


Thus, the speed of the stone when it hits the ground is 20m/s.


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