WAEC Physics 2021 Paper

Concept:The time taken to fall to the ground depends only on the vertical motion.Horizontal velocity does not affect the time of fall.Explanation:The stone is thrown horizontally, so its initial vertical velocity is $u = 0\,\mathrm{m/s}$.The vertical displacement is the height of the tower: $s = 20\,\mathrm{m}$.The acceleration due to gravity is $g = 10\,\mathrm{m/s^2}$.Use the equation of motion for vertical fall:$$s = ut + \frac{1}{2}gt^2$$Substitute the known values:$$20 = 0 \cdot t + \frac{1}{2}(10)t^2$$$$20 = 5t^2$$$$t^2 = \frac{20}{5} = 4$$$$t = \sqrt{4} = 2\,\mathrm{s}$$The horizontal motion is irrelevant for calculating this fall time.Answer:The stone takes $2\,\mathrm{s}$ to reach the ground.Therefore, the correct option is A. $2\,\mathrm{s}$.