1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Sunlit earth

  1. Mar 3, 2010 #1
    1. The problem statement, all variables and given/known data
    At Earth’s location, the intensity of sunlight is 1.5 kW/m^2. If no energy escapes
    Earth, by how much would Earth’s mass increase in a day? (Radius of Earth is
    6.37 × 10^6 m.)

    3. The attempt at a solution

    ok so, at any one time, half of the earth is lit.

    so surface area lit = 1/2 x 4[tex]\Pi[/tex]r2 =

    intensity = 1500 W/m2

    so energy per second on earth = 1500 x 2.55x1014 = 3.82 x 1017 J

    energy per day = 3.82 x 1017 x 60 x 60 x 24 = 3.3 x 1022

    so E = mc2
    so m = E / c2 = 3.67 x 105 kg

    but the correct answer is half of this. so where did i go wrong? thanks for help!
  2. jcsd
  3. Mar 3, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You are treating the light as being perpendicular to the surface area of the sphere, but it's not. The light comes in parallel. (Treat the earth as a disk, not a sphere.)
  4. Mar 3, 2010 #3
    ooo! thanks a lot doc! now i remembered my lecturer said this too !
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook