Mister Underhill, oh equus straddling wordsmith of such withering countenance, I guide your attention to Fëanor himself, at that time in Valinor when Finwe heaved a Silmaril at him in disgust.
More importantly, with reference to thermodynamics. The Hobbit was published in the 1930s, by which time the laws of physics, including those of friction, air resistance and the displacement effect of a foreign body, were much as we know them today. Tolkien, as an eminently bright man, would have known this and applied them thus.
Therefore I quibble with your post-Hobbit non-canonical status application with reference to twentieth century thermodynamics in the matter of giants throwing rocks in LoTR.
[ August 08, 2002: Message edited by: Stephanos ]