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Homework Help: Radiation Pressure Force on Earth

  1. Apr 25, 2013 #1
    1. The problem statement, all variables and given/known data
    The intensity of sunlight reaching the earth is [tex] 1360 \frac{W}{m^2}[/tex].
    a.) Assuming all sunlight is absorbed, what is the radiation pressure force on the earth? Give your answer in Newtons. [tex]F_r=[/tex]
    b.) Give your answer as a fraction of the suns gravitational force on the earth. [tex] \frac{F_r}{F_g}= [/tex]

    2. Relevant equations
    Intenisty is given by [tex] I= \frac{P}{A}[/tex] where P power and A is the cross sectional area.
    Force is given by [tex]F=\frac{P}{c}=\frac{IA}{c}[/tex] where c is the speed of light in a vacuum.

    3. The attempt at a solution
    I solved the first part correctly, no problem. I got that the radiation pressure force was [tex] 5.78*10^8 N[/tex]
    For the second part, I simply divided this number by the gravitational force of the sun on the earth, which I found to be [tex] F_g=3.53*10^{22} N [/tex]
    When I divided these two numbers, I got [tex]\frac{F_r}{F_g}= 1.64*10^{-14}[/tex] which of course should have no units. However, the problem asks for units, so I put in Newtons anyways, and I was told the my answer must have dimensions of percent (I wasn't aware a unitless quantity had dimensions). So, I slapped in % sign. I was then told to try again; that I was wrong. What am I missing?
  2. jcsd
  3. Apr 25, 2013 #2


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    2017 Award

    Staff: Mentor

    A fraction of 1 would correspond to 100%. A fraction of 1.64*1014 corresponds to...?
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