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.
A. `2.1xx10^(-6)N`
B. `1.2xx10^(-6)N`
C. `1.2xx10^(6)N`
D. `2.1xx10^(6)N`

Answer 1

Correct Answer - B
The total energy falling on the surface is `U=18xx20xx(30xx60)=6.48xx10^(5)J`
Therefore, the total momentum delivered (for complete absorption is)
`p=(U)/(c)=(6.48xx10^(5))/(3xx10^(8))=2.16xx10^(-3)"kg m s"^(-1)`
The average force exerted on the surface is
`F=(p)/(t)=(2.16xx10^(-3))/(0.18xx10^(4))=1.2xx10^(-6)N`