Question

Light with an energy flux of $18W/c{m}^2$ falls on a non-reflecting surface at normal incidence. The surface has an area of $20c{m}^2$, then the total momentum delivered on the surface during a span of $30$ minuts is

• A. $1.2\times {10}^{-6}{kgms}^{-1}$
• B. $2.16\times {10}^{-3}{kgms}^{-1}$
• C. $1.18\times {10}^{-3}{kgms}^{-1}$
• D. $3.2\times {10}^{-3}{kgms}^{-1}$

Answer 1

The correct option is A $1.2\times {10}^{-6}{kgms}^{-1}$

Total energy falling

$U=\left(18W/c{m}^2\right)(30\times 60s)\left(20c{m}^2\right)$

$=6.48\times {10}^{5}J$

Total, momentum delivered is

$P=\frac{U}{c}=\frac{6.48\times {10}^5}{3\times {10}^8}=2.16\times {10}^{-3}kg-m/s$