Light with an energy flux of 18 W cm^-2 falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 cm², the total momentum delivered to the surface during a span of 30 minutes is:

$2.16 \times 10^{-3} ~kg ~ms^{-1}$

Total energy falling on the surface = Energy flux × time × area

$\Rightarrow \mathrm{U}=\left(18 \mathrm{~W} \mathrm{~cm}^{-2}\right) ×(30 × 60 s) ×\left(20 \mathrm{~cm}^{-2}\right)=6.48 × 10^5 J$

Total momentum delivered to the surface $\mathrm{p}=\frac{\mathrm{U}}{\mathrm{c}}=\frac{6.48 × 10^5}{3 × 10^8}=2.16 × 10^{-3} kg ~ms^{-1}$