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I was given an exercise to make a simple model that suggests that the age of the Earth is 78000 years old.

My current starting point is

$$u_{tt} = u_{xx}$$

so a wave equation. So in polar coordinates:

$$u_{tt} = u_{rr} + \left( \frac{1}{r}\right) u_{r}$$

So the calculation is easier. I should now use the result by assuming that the centre of the Earth consists only of iron. Hint to the problem is "iron balls or iron cannons, Buffon".

How would you construct such a model?

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...something here generally about the issue. – hhh Oct 9 '11 at 22:59
You may want to reconsider your starting place...and heat flow is usually represented with a slightly different formalism than wave motion. – dmckee♦ Oct 9 '11 at 23:13
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Hi Masi - after some consultation, this seems to fall under the category of "do-my-homework" type questions that are prohibited in our FAQ. Basically, you're not asking about a specific physical concept, which is the kind of question that would be appropriate for this site. Plus, you haven't really done much on your own to narrow down what is giving you trouble with the question. See our meta post on homework-type questions for more information. If you'd like advice on improving the question, feel free to ask in Physics Chat. – David Zaslavsky♦ Oct 10 '11 at 5:00
P.S. The differential equation you wrote is not a diffuse equation, it's a wave equation, since it's about 2nd derivative WRT time – valdo Oct 10 '11 at 15:49

closed as off topic by David Zaslavsky♦ Oct 10 '11 at 4:52

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