A stone is dropped from a height of 10cm above the top of a window 80 cm high. The time taken by the stone to cross the window is (g = 9.8 m${\mathrm{s}}^{–2}$)

$$\begin{gathered} S = ut+\frac{1}{2}a{t}^2 \\ where, u = 0 we get \\ 10 cm = \frac{1}{2}\times 10\times t^2 \\ \frac{10}{100}= \frac{1}{2}\times 10\times t^2 \\ {t}^2 = \frac{2}{100} \\ t = \frac{\sqrt{2}}{10}=\frac{1.414}{10}=0.14 = \frac{1}{7}sec \\ \frac{90 }{100} = \frac{1}{2}\times 10\times t^2 \\ \frac{18}{100}=t^2, t = \frac{4.25}{10} = 0.428 = \frac{3}{7} \\ = t_2-t_1 = \frac{3}{7}-\frac{1}{7} = \frac{2}{7} \end{gathered}$$