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.

Explanation

The total energy falling on the surface is
`U=(18W//cm^(2))xx(20cm^(2))xx(30xx60)`
`=6.48xx10^(6)J`
Therefore, the total momentum dilivered (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))/(0.18xx14^(4))=1.2xx10^(-6)N`