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 exrted on the surface during a 30 minute time span.

Explanation

The total energy falling on the surface is `U = (18 W // cm^(2) ) xx (20cm^(2) ) xx (30 xx 60) = 6.48 xx 10^(5) J` Therefore, the total momentum delivered (for complete absorption) is
`P= (U)/( c)=(6.48 xx 10^(5) J)/(3xx10^(8)m//s) = 2.16xx10^(-3) kg m//s`
The average force exerted on the surface is `F = (P)/(t ) = (2.16 xx10^(-3))/(0.18xx10^(4)) = 12xx10^(-6)N`