Question

Light with an energy flux of $18w/c{m}^2$ falls on a non-reflecting surface at normal incidence. If the surface has an area of $20c{m}^2$. Find the average force exerted on the surface during a 30 minute time

• A. $1.2\times {10}^{-6}N$
• B. $2.4\times {10}^{-6}N$
• C. $2.16\times {10}^{-3}N$
• D. $1.5\times {10}^{-6}N$

Answer 1

The correct option is A $1.2\times {10}^{-6}N$

The total energy falling on the surface is $U=\left(18W/c{m}^2\right)\times \left(20c{m}^2\right)\times \left(30\times 60\right)=6.48\times {10}^{5}J$

therefore$,$ the total momentum delivered is

$P=\frac{U}{c}=\frac{\left(6.48\times {10}^{5}J\right)}{\left(3\times {10}^{8}m/s\right)}=2.16\times {10}^{-3}kgm/s$

The average force exerted on the surface is

$F=\frac{p}{t}==\frac{2.16\times {10}^{-3}}{0.18\times {10}^4}=1.2\times {10}^{-6}N$

Hence,

option $\left(A\right)$ is correct answer.