Light with energy flux of 18 W/cm² falls on a non reflecting surface of area 20 cm² at normal incidence. The momentum delivered in 30 minutes is

Energy flux = 18 Wcm^–2 = 18 x10⁴  Wm^-2

Total energy incident

U = flux x Area x time

   = 18 x10⁴ x 20 x 10^-4 x 30 x 60 = 18 x 36 x 10³

Momentum delivered

$\mathrm{P}=\frac{\mathrm{U}}{\mathrm{C}}=\frac{18 \mathrm{x} 36 \mathrm{x} {10}^3}{3 \mathrm{x} {10}^8}=2.16 \mathrm{x} {10}^{-3} \mathrm{kg} {\mathrm{ms}}^{-1}$