A child accidentally drops a toy from his roof. The toy's final velocity before hitting the ground was 9m/s. Determine the building's height. 

The toy's acceleration is equal to gravity's acceleration.

$\mathrm{a}=\mathrm{g}=9.8 \mathrm{m}/{\mathrm{s}}^2$

Applying the third equation of motion, $v^2=u^2+2as$

$9^2=0 + 2\times 9.8 \times s$

$s= 4.13 m$

Hence, the height of the building must be  4.13 metres