Light with an energy flux of `18 W//cm^(2)` falls on a non-reflacting surface at normal incidence. If the surface has an area of `20 cm^(2)`, find the average force exerted on the surface during a 30 minute time span.

Explanation

The total energy falling on the surface is `U=18xx20xx30xx60=6.48xx10^(5)J`
Therefore, the total momentum delivered (for complete absorption) is
`p=(U)/(c )=(6.48xx10^(5)J)/(3xx10^(8)m//s)=2.16xx10^(-3)` kg m/s
The average force exerted on the surface is `F=(p)/(t)=(2.16xx10^(-3))/(0.18xx10^(4))=1.2xx10^(-6)N`.