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Homework Help: Numerical Estimation problem

  1. Aug 21, 2010 #1
    1. The problem statement, all variables and given/known data

    "Estimate the mass of water in all the World’s oceans."


    2. Relevant equations

    I know the following:
    Two-thirds of the earth is sea.
    The density of seawater is 1025 kg/m3.
    Radius of the earth = 6.3*106m.


    3. The attempt at a solution

    Let's assume that the earth's crust is a spherical shell that has a thickness of 0.001 times the radius of the earth.
    So, the volume of the crust = (4[tex]\pi[/tex]r2)(router-rinner) = 3.1*1018 m3.
    So, volume of seawater = 2.1*1018 m3.
    So, mass of seawater = 2.1*1021 kg.

    Is the answer reasonable? How might I improve it?
     
  2. jcsd
  3. Aug 21, 2010 #2
    no no, ~2/3 of the SURFACE of the earth is ocean. much much less of the entire earth is ocean. I'd recommend using the average depth of the ocean, and the surface area of the earth.
     
  4. Aug 21, 2010 #3
    "no no, ~2/3 of the SURFACE of the earth is ocean. much much less of the entire earth is ocean.": I am reading the 'surface' as the crust (or the outer layer) of the earth. I think that's what I have used in the calculation, not ~2/3 of the entire volume of the earth.

    " I'd recommend using the average depth of the ocean, and the surface area of the earth. ":
    I think that's what I did above.

    I'd be glad if you could offer some genuine help.
     
  5. Aug 21, 2010 #4
    Very sorry! I took, "Two-thirds of the earth is sea" to mean that you thought that 'two-thirds of the earth was sea.' I clearly should have known you meant something completely different.

    Your numbers look good; your radius is a little big, but your density a little small (for sea-water), you should be within a factor of two.
     
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