Suppose that earth was formed as molten rock at some very high temperatures. Estimate the time for the earth to cool to 300 K.
(Note: This calculation was first performed by Lord Kelvin. The cooling time determined by Kelvin was significantly smaller than the geological evidence for the age of the earth. This generated a great scientific debate. The problem was resolved with the discovery of radioactive elments in the earth. The energy released in radioactive decays increases the cooling time.)