# Question #189b5

Feb 22, 2017

Let downward initial velocity of the pebble be $u$

So utilising $h = u \times t + \frac{1}{2} \times g \times {t}^{2}$ we get

$91.2 = u \times 4 + \frac{1}{2} \times 9.8 \times {4}^{2}$

$\implies 4 u = 91.2 - 8 \times 9.8$

$\implies u = \frac{91.2}{4} - 8 \times \frac{9.8}{4}$

$\implies u = 22.8 - 19.6 = 3.2 \text{m/s}$