Q6 of 25 Page 130

A cricket ball is dropped from a height of 20 m. (a) calculate the speed of the ball when it hits the ground? (b) Calculate the time it takes to fall through this height? [g = 10m/s2]

Initial velocity = u = 0m/s

Height = h = 20m


(a) For final velocity = v


Using 3rd equation of motion


v2 = u2 + 2gh


v2 = 02 + 2(10 ms–2)(20m)


v2 = 400m2s–2


⇒ v = 20m/s


(b) For time


Using 1st equation of motion


v = u + gt


20 = 0 + 10(t)


⇒ time = 2 seconds.


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