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Homework Help: Find force from flux

  1. Oct 19, 2013 #1
    1. The problem statement, all variables and given/known data

    What is the flux on a solar sail per square meter, if it orbits the Sun at 3 AU. Flux at 1 AU is 1.36 KW/square meter

    2. Relevant equations

    Flux goes as 1/r^2

    3. The attempt at a solution

    Flux at 3 AU will be 1.36/3^2 = 1.36/9 = 0.15 KW/square meter

    I can't figure out how to relate this to the force though?

    Any help would be appreciated.

    *This is practise for a test not an assignment queston
  2. jcsd
  3. Oct 19, 2013 #2


    User Avatar

    Staff: Mentor

    The force will depend upon the angle of incidence of the light (assuming we are talking photon flux here!) and how much of the light is absorbed versus reflected. For perfect reflection and an angle of incidence of zero degrees from the normal the pressure exerted will be 2Ef/c. If the angle is not zero degrees then there's a cos2 term. Look up "radiation pressure".
  4. Oct 20, 2013 #3
    The information I gave in the question is all I have.
  5. Oct 20, 2013 #4
    Oops just realized I put the wrong thing in the first post, it should say "What is the force..."
  6. Oct 21, 2013 #5
    I think I got it. Force per square meter = Pressure and (Radiative) Pressure = Flux/c
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