Light with an energy flux of $16W/c{m}^2$  falls on a non reflecting surface at normal incidence. If the surface has an area of $20 c{m}^2$  , find the average force exerted on the surface during a 30 minute time span.

The total energy falling on the surface is
$U=16\times 20\times 30\times 60$
$5.76\times {10}^{5}J$
        The total momentum delivered (for complete absorption) is
$p=\frac{u}{c}=\frac{5.76\times {10}^5}{3\times {10}^8}$
$\frac{5.76}{3}\times {10}^{-3}$
$=1.92\times {10}^{-3}$
The average force exerted$=\frac{p}{t}=\frac{1.92\times {10}^{-3}}{0.18\times {10}^4}=10.66\times {10}^{-7}$
                                                     $=1.06\times {10}^{-6}$