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Light with an energy flux of 18 W//cm^(2) falls on a non-reflecting surface at normal incidence. I f the surface has an area o f 20 cm^(2), find the average force exerted on the surface during a 30 minute time span. |
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Answer» <P> Solution :The total energy falling on the surface is `U = (18 W // cm^(2) ) xx (20cm^(2) ) xx (30 xx 60) = 6.48 xx 10^(5) J` Therefore, the total momentum delivered (for complete absorption) is`P= (U)/( c)=(6.48 xx 10^(5) J)/(3xx10^(8)m//s) = 2.16xx10^(-3) kg m//s` The average FORCE EXERTED on the surface is `F = (P)/(t ) = (2.16 xx10^(-3))/(0.18xx10^(4)) = 12xx10^(-6)N` |
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