Question

A stone is dropped from a certain height and reaches the ground is 5 second . If the stone is stopped after 3 second of its fall and then allowed to fall again then the time taken by the stone to reach the ground after covering the remaining distance is _______.

Answer 1

Distance covered by stone in 5s is given by 1/2*g*t^2 = 125m (assuming g=10m/s^2)
So now the stone is allowed to fall for 3 second which means distance covered would be 45m. Again the stone is allowed to fall
So,
$\frac{1}{2} \times g {t}^{2} = 125 - 45 = 80 \\ 10 \times {t}^{2} = 80 \times 2 = 160 \\ {t}^{2} = 16 \\ t = 4$
So the time taken is 4 second.