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Light with an energy flux of `18 W//cm^2` falls on a non-reflecting 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» The total energy falling on the surface is `U=(18W//cm^(2))xx(20cm^(2))xx(30xx60)` `=6.48xx10^(6)J` Therefore, the total momentum dilivered (for complete absorption) is `p=(U)/(c)=(6.48xx10^(5)J)/(3xx10^(8)m//s)=2.16xx10^(-3)kg m//s` The average force exerted on the surface is `F=(p)/(t)=(2.16xx10^(-3))/(0.18xx14^(4))=1.2xx10^(-6)N` |
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