Light with an energy flux of 20 W/cm² falls on a non-reflecting surface at normal incidence. If the surface has an area of 30cm², the total momentum delivered (for complete absorption) during 30 minutes is

Energy flux $\varphi =20\mathrm{W}/{\mathrm{cm}}^2$

Area A= 30cm², time t=30 x 60 sec
U = Total energy falling in t sec= Energy flux$\times \text{ Area }\times \text{ time }=\varphi \text{ At }$
U= 20x 30x30x60 J
$\mathrm{Momentum} \mathrm{of} \mathrm{the} \mathrm{incident} \mathrm{light}$
$=\frac{u}{c}=\frac{20\times 30\times 30\times 60}{3\times {10}^8}=36\times {10}^{-4} {\mathrm{kgms}}^{-1}$
As no reflection from the surface and for complete absorption& momentum of reflected radiation is zero  Momentum delivered to surface = Change in momentum
$\begin{array}{l}={\mathrm{p}}_{\mathrm{f}}-{\mathrm{p}}_{\mathrm{i}}=0-36\times {10}^{-4}\mathrm{kgm}/\mathrm{s}\\ =-36\times {10}^{-4}\mathrm{kgm}/\mathrm{s}\end{array}$
(-) sign shows the direction of momentum,