Answers and comments for the review activities and problems in Chapter 5.

Most of the questions in this chapter are discussion questions for which it is difficult to give simple answers. I particularly like GR 6 which deals with an old conundrum. It emphasizes the importance of definitions, or, if you like, the importance of knowing what you're talking about. Once you decide on the relevant definition the question becomes simple. The point is similar to that on the meaning of 'reality' on page 96.


1. Is the sun in fact farther from the earth in winter?

2. Coulomb's law describes the force between two charges that take up no space ("point charges"). It is an empirical law. No experiment has ever shown any limits to its accuracy.

Compare this to the case of Newton's law of gravitation: it is also an empirical law, namely that for the force between two point masses. We know, however, that the results of Einstein's general theory of relativity supersede those of Newton.

4. No.

6. Be careful to think whether a statement you make (or one in the problem) is theoretical or empirical.


1. There is more on the Greenhouse effect on page 332.

5. The more definite you want your answer to be, the more you will need to expose yourself (or other guinea pigs) to the effect that you want to investigate!