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 ms^–2)

v = u + at ; v² - u² = 2as

u = 0; v² = 2gs

v² = 2 x 9.8 x 10 x 10^-2

v = 1.4ms^-1 ; (At top of window)

v² = 2 x 9.8(80 + 10)10^-2 ; v¹ = 4.2ms^-1

Since v = u + at ; v¹ = v + gt

4.2 = 1.4 + 9.8t$\Rightarrow$t = 2/7s