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The chapter you cite explains the principle of equivalence quite well. I now realize I have been forgetting the most important aspect of the principle, namely that the inertial mass is equivalent to the gravitational mass.

In other words, Newton's second law, which Einstein would write F=dp/dt (force equals rate of change of momentum), has a term, momentum, p, which depends on inertial mass, m, and velocity, v. (p=mv is a very good approximation at speeds much less than c, the speed of light.)

The gravitational force on an object also depends on its mass -- in this case the gravitational mass. If you think about it, the two masses are not necessarily the same. There could be a gravitational equivalent of electric charge that is unrelated to the inertial mass.

The principle of equivalence states that these two masses are indeed the same, and Einstein's thought experiment, so clearly described in that chapter, uses that to develop the equivalence of gravity and an accelerated frame of reference.

By the way, getting back to the original point of this thread, the curvature of light in a gravitational field is explained with equal clarity in Chapter 22. (http://www.bartleby.com/173/22.html) For some reason, Don has a hard time accepting that as valid and proposes a different mechanism that doesn't fit with observations, such as the Solar eclipse data cited by Einstein and the additional evidence, such as gravitational lensing, which is available to us today.

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)