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 30 cm², then the total momentum delivered (for complete absorption) during 30 min is:

36 × 10^-4 kg-m/s

Given, energy flux, $\phi=20 W/cm^2$

Area, $A = 30 ~cm^2$

Time, t = 30 min = 30 × 60s

Now, total energy falling on the surface in time $t$ is, $u=\phi At=20 \times 30 \times(30 \times 60)$ J

Momentum of the reflected light = 0

∴  Momentum delivered to the surface = $36 \times 10^{-4}-0=36 \times 10^{-4}$ kg-m/s