Question

Light with an energy flux of `18 W//cm^2` falls on a non-reflecting 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.

Answer 1

Total energy falling on the surface is `U=Pat`
`U=(18xx10^(4))(20xx10^(-4))(30xx60)=6.48xx10^(5)J`
Total momentum delivered (complete absorption)
`p=(U)/(C )=(6.48xx10^(5))/(3xx10^(8))=2.16xx10^(-3)kg m//sec`
average force exerted
`F=(P)/(t)=(2.16xx10^(-3))/(30xx60)=1.2xx10^(-6)N`