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Light with an energy flux of 18 \mathrm{~W} \mathrm{~cm}^{-2}  falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 \mathrm{~cm}^2, the total momentum delivered to the surface during a span of 30 minutes is:
 

Option: 1

2.16 \times 10^{-3} \mathrm{~kg} \mathrm{~ms}^{-1}
 


Option: 2

2.16 \times 10^{-4} \mathrm{~kg} \mathrm{~ms}^{-1}
 


Option: 3

4.32 \times 10^{-3} \mathrm{~kg} \mathrm{~ms}^{-1}
 


Option: 4

4.32 \times 10^{-4} \mathrm{~kg} \mathrm{~ms}^{-1}


Answers (1)

best_answer

\mathrm{{Total\: energy\: falling \: on\: the\: surface = Energy\: flux \times time \times area}}

\mathrm{\Rightarrow \mathrm{U}=\left(18 \mathrm{Wcm}^{-2}\right) \times(30 \times 60 \mathrm{~s}) \times\left(20 \mathrm{~cm}^{-2}\right)=6.48 \times 10^5 \mathrm{~J}}

Total momentum delivered to the surface

\mathrm{\mathrm{p}=\frac{\mathrm{U}}{\mathrm{c}}=\frac{6.48 \times 10^5}{3 \times 10^8}=2.16 \times 10^{-3} \mathrm{~kg} \mathrm{~ms}^{-1}}

Hence option 1 is correct.

Posted by

Ritika Harsh

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