WAEC 2020 · Paper 2 · Q8

Kinematics & dynamics

A ball is thrown vertically upwards with a velocity of $10\ \text{m s}^{-1}$ from a point $75$ metres above the ground. How long will it take the ball to strike the ground? $[\text{Take } g = 10\ \text{m s}^{-2}]$

(a)
Time in seconds

Try it on a graph

Height above ground h = 75 + 10t − 5t² (x-axis is time t). Where does it hit the ground? Where is it highest?

Worked solution (try it first)

Take up as positive: $u = 10$ , $a = -10$ , and the ground is $75\text{ m}$ below, so $s = -75$ .
$s = ut + \frac12at^2$ : $-75 = 10t - 5t^2$ , so $t^2 - 2t - 15 = 0$ .
$(t - 5)(t + 3) = 0$ , so $t = 5\text{ s}$ (time cannot be negative).

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