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Question

A stone is dropped from a height of 20 m.
(a) How long will it take to reach the ground?
(b) What will be its speed when it hits the ground? (g = 10 m/s2)

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Solution

(a) Distance travelled by the stone while falling from the height 20 m to the ground = 20 m

The initial velocity of the stone = 0

Acceleration of the stone = g

By using the second equation of motion, we can write, s=ut+12gt2

Here, u = 0, s = 20 m, and 't' is the time taken by the stone to reach the ground.

So,

20 =0 + 12×g×t2t2=40g=4010=4t=4 s

(b) Using the first equation of motion, we can write, v = u + gt (Where 'v' is the speed of the stone when it hits the ground)

So, v = 0 + 10×4=40 m/s




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