WAEC 2020 · Paper 1 · Q20

Kinematics & dynamics

A stone was dropped from the top of a building 40 m high. Find, correct to one decimal place, the time it took the stone to reach the ground. [Take $g = 9.8\text{ m s}^{-2}$ ]

Worked solution (try it first)

Dropped means it starts from rest, so use $s = ut + \frac{1}{2}gt^2$ with $u = 0$ : $40 = 4.9t^2$ .
Divide by 4.9: $t^2 = 8.163$ .
Take the square root: $t = 2.857$ , which is 2.9 s to one decimal place, option A.

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