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    Light with an energy flux of 18 W/cm2\text{cm}^2cm2​ falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 cm2\
    Question

    Light with an energy flux of 18 W/cm2\text{cm}^2​ falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 cm2\text{cm}^2​, find the total momentum delivered (for complete absorption) during a 30 minute time span?

    A.

    2.16×105 kg/s2.16 \times 10^{-5} \mathrm{~kg} / \mathrm{s}​​

    B.

    2.16×102 kg/s2.16 \times 10^{-2} \mathrm{~kg} / \mathrm{s}​​

    C.

    2.16 kg m/s

    D.

    ​​​2.16×103 kgm/s 2.16 \times 10^{-3}\, \text{kg} \cdot \text{m/s}​​

    Correct option is D

    Given: Energy flux =18 W/cm2 Area A=20 cm2=20×104 m2 Time t=30 minutes=1800 s Speed of light c=3×108 m/sPower incident: P=Energy flux×AreaMomentum delivered: p=PtcP=18×104×20×104=360 Wp=360×18003×108=2.16×103 kgm/s\textbf{Given:} \\\quad \bullet\ \text{Energy flux } = 18\, \text{W/cm}^2 \\\quad \bullet\ \text{Area } A = 20\, \text{cm}^2 = 20 \times 10^{-4}\, \text{m}^2 \\\quad \bullet\ \text{Time } t = 30\, \text{minutes} = 1800\, \text{s} \\\quad \bullet\ \text{Speed of light } c = 3 \times 10^8\, \text{m/s} \\\\\quad \text{Power incident: } P = \text{Energy flux} \times \text{Area} \\\quad \text{Momentum delivered: } p = \frac{P \cdot t}{c} \\\\\quad P = 18 \times 10^4 \times 20 \times 10^{-4} = 360\, \text{W} \\\quad p = \frac{360 \times 1800}{3 \times 10^8} = 2.16 \times 10^{-3}\, \text{kg} \cdot \text{m/s}​​

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