Light with energy flux of $18W/c{m}^2$ falls on a non reflecting surface at normal incidence. If the surface has an area of $20c{m}^2$. The average force exerted on the surface during a 30 minute time span is about. 

The toal energy falling on the surface is
$$\begin{gathered} U=\left(18\mathrm{w}/{\mathrm{cm}}^2\right)\times \left(20{\mathrm{cm}}^2\right)\times (30\times 60) \\ =6.4\times {10}^{5}J \end{gathered}$$

Total momentum delivered (for complete absorption) is $\frac{6.4\times {10}^5}{3\times {10}^8}=2.16\times {10}^{-3}kgm{s}^{-1}$

 $F=\frac{Momentumdelivere}{time}=\frac{2.16\times {10}^{-3}}{30\times 60}$

$=1.2\times {10}^{-6}N$