A plank of mass m is lying on a rough suface having coeffient of frinction as `mu` is situation as shown in fig Find the acceleration of the plank assuming that it slips and the surface of body exposed to radiaton is black body.

Explanation

KEY IDEA
First of all let us find the energy received per second by surface exposed to radiation. The area of the surface is ab. But the area of across section of the beam incident on the surface is not ab. From the figure, the area of cross section of the beam is ab `cos theta`.
Calculation : So, the power of the incident beam is
`P=Iab cos theta`
The number of photons incident per unit time can be found by
`P=(dn)/(dt)xx(hc)/(lambda)`
So,
`(dn)/(dt)=(Iab cos thetalambda)/(hc)`

A light beam incident on a black body at an angle `theta` to the normal.
Each of the photon will be completely absorbed by the surface. This will produce an impulse in the normal direction will be absorbed by the surface on which it is kept.
Radiation force experience by surface is
`F=(dn)/(dt)xxp`
where p is the momentum of the photon incident. By de Broglie.s hypothesis,
`p=(h)/(lambda)`,
`F=(Iab cos theta)/(c)`