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The average energy flux of sunlight is $1\: kW/m^2$. This energy is falling normally on the plate surface of area $10\: cm^2$ which completely absorbs the energy. How much force is exerted on the plate if it is exposed to sunlight for 20 minutes?

$\begin {array} {1 1} (a)\;33 \times 10^{-8}N & \quad (b)\;3.3 \times 10^{-8}N \\ (c)\;33 \times 10^{-9}N & \quad (d)\;3.3 \times 10^{-9}N \end {array}$

Total energy falling on the plate, $U =$ energy flux $\times$ Area $\times$ time
$= 1000 \times 10 \times 10^{-4} \times 20 \times 60 = 1200 J$
Momentum, $p = \large\frac{U}{c }$$= \large\frac{1200}{3}$$\times 10^8 = 400 \times 10^{-8}kgm/s$
Force, $F = \large\frac{p}{time }$$= \large\frac{400 \times 10^{-8}}{1200}$$ = 3.3 \times 10^{-9}N$
Ans : (d)