A light with an energy flux of $18 \mathrm{~W} \mathrm{~cm}^{-2}$ falls on a non-reflecting surface at…
A light with an energy flux of $18 \mathrm{~W} \mathrm{~cm}^{-2}$ falls on a non-reflecting surface at normal incidence. If the surface has an area of $20 \mathrm{~cm}^2$, the average force exerted on the surface during a 30 minute time span is ____ $\left(c=3 \times 10^8 \mathrm{~ms}^{-1}\right)$.