Question

A stone is thrown directly upward with an initial speed of 4 m/s from a height of 20 m. After what time interval does the stone strike the ground

Answer 1

Given :

( Let , take upward direction +ve and downward direction -ve )

Initial speed of stone , u = 4 m/s .

Height , h = 20 m .

To Find : Time taken to reach ground .

Solution :

We know , by equation of motion .

Displacement is given by :

$h=h_o+ut+\dfrac{gt^2}{2}\\\\0=20+4t-2t^2\\\\t^2-2t-10=0$  ( Here , g = acceleration due to gravity = $-9.8\ m/s^2$ .)

Solving above equation , we get :

t = 4.32 s .

Hence , this is the required solution .