# Homework Help: Estimating problem

1. Sep 2, 2006

### rleung3

Hi,

I know this question sounds so simple, but I can't figure out how to solve it. The question is the following: Within an order of magnitude, how many drops of water are in all the oceans on earth? (Take an average depth of the ocean about 10 km. Assume that contains 25 drops of water.)

Without knowing the volume of all the oceans in the world, I don't see how I can come up with an answer that is within an order of magnitude. My approach would be to multiple 25 drops/cm^3 by the volume of the oceans (in cm^3). I went online to look up the actual volume, but the answer turns out incorrect (the online program I am using doesn't tell me the correct answer, but it tells me when I am NOT ocrrect).

I also tried to take (25 drops/cm^3) ^ (1/3) to finds the drops^(1/3) per cm, after which I multiplied that figure by the depth of the ocean (in cm) to get the drops^(1/3), and then cubing that to gets the number of drops. That answer also turned out incorrectly.

Is there any other direction I should take? Thanks so much! :)

Ryan

2. Sep 2, 2006

### J Hann

If you're given an average depth of 10 km you could try using the fact
that the oceans cover about 71% of the surface of the earth. The radius varies from 6.378 E6 m at the equator to 6.356 E6 m at the poles. Also, the radius of a sphere of a sphere with the same volume = 6.371 E6 m.

3. Sep 2, 2006

### arunbg

You do know the radius of the earth, don't you ? What's the curved surface area for a sphere like the earth (app.) in terms of r ? Oceans cover 3/4 th of the earth' crust (app.)
Multiply area with depth gives you ....

4. Sep 3, 2006

### rleung3

Hmmm....I tried it your way, but I don't seem to get the right answer. It says here that the answer does not have anything to do with scientific notation??? I don't see how that is right...