
#1
Oct1905, 11:21 AM

P: 65

Ok, i got a problem that reads as followed.
Suppose that a perfectly reflecting circular mirror is initially at rest a distance R away from the sun and is oriented so that the solar radiation is incident upon, and perpendicular to, the plane of the mirror. What is the critical value of mass/area for which the radiation pressure exactly cancels out the force due to gravity? Ok so lets start with the given:, I know mass of the sun is 2.0 x 10^30 kg intensity of sunlight as a function of the distance R from the sun = 3.2x10^25(1/R^2) (w/m^2) and the gravitational constant is 6.67x 10^11 Radiation pressure ecerted on a perfectly reflecting surface is P= 2S/c where C is the speed of light and S is the poynting vector? I know the answer is going ot be mass/area in which the mass is mass of the sun. The answer will also be in kg/m^2. Now i am not sure how to setup this problem. 



#2
Oct1905, 02:39 PM

Sci Advisor
HW Helper
P: 6,567

[tex]P = F/A = \frac{\rho*Ad*GM_{sun}}{Ar^2} = \frac{\rho*d*GM_{sun}}{r^2}[/tex] Equating the two: [tex]P = \Phi_E/c = \frac{\rho*d*GM_{sun}}{r^2}[/tex] where [itex]\Phi_E = \frac{E}{4\pi r^2}[/itex] is the energy flux (E/A) You should be able to work it out from that. AM 


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