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Light with an energy flux of 18 W//cm^(2) falls on a nonreflecting surface at normal incidence. If the surface has an area of 20 cm^(2), find the average force exerted on the surface during a 30 minute time span. |
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Answer» Solution :The TOTAL energy falling on the surface is `U=(18 W//cm^(2))xx(20 cm^(2))xx(30xx60 s)` `=6.48 xx10^(5) J` Therefore, the total momentum DELIVERED (for complete absorption) is `p=(U)/(c )=(6.48xx10^(5) J)/(3xx10^(8) m//s)=2.16 xx10^(-3) kg m//s` The AVERAGE force exerted on the surface is `F=(p)/(t) =(2.16 xx10^(-3)/(0.18xx10^(4))=1.2xx10^(-6) N` How will your result be modified if the surface is a perfect reflector? |
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