via Wikimedia Commons.
A friend of mine on Twitter, Aaron (a pseudonym), is an overseas, 16-year old boy, who seriously admires Albert Einstein and wants to become a physicist. He continually writes me (Ted, also a pseudonym) questions about the theory of relativity and related topics, and I am sending answers. In this series of blog posts, those questions and answers are reproduced with modifications. I am not an expert in the fields of physics related to relativity. So, my answers might contain errors. If you find any error, please do not hesitate to write a comment for the benefit, not only of the boy and me, but also of other readers.
Aaron: It seems that Newton did not know exactly how gravity worked. Then, there came Einstein to find the answer. Why do they still write about Newton's law of gravity in textbooks?
Ted: Newton's law of gravity and equation of motion are highly accurate approximations to Einstein's general and special theories of relativity. Einsteinian mechanics did not make Newtonian mechanics useless but expanded the scope of the latter. The latter is quite simple and yet is useful for doing calculations of the motion of materials being situated in a weak gravitational field and having a speed much smaller than the speed of light. Therefore, we first study Newtonian mechanics at high schools.
Note Added later: An educational blog post on the relation between Einstein's relativity and Newton's mechanics has appeared: Matt Strassler, "How did Einstein do it?" Blog site Of Particular Significance (2012).
(Originally written on February 24, 2011)