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=(18 W//cm^(2))xx(20 cm^(2))xx(30xx60)`
`=6.48 xx 10^(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`