  For generalized coordinate systems, the numerical values of vector and tensor
components do not generally equal the physical values, i.e., the values one
would measure with standard physical instruments. Hence, calculating physical
components from coordinate components is important for comparing experiment
with theory. Surprisingly, however, this calculational method is not widely
known among physicists, and is rarely taught in relativity courses, though it
is commonly employed in at least one other field (applied mechanics.) Different
derivations of this method, ranging from elementary to advanced level, are
presented. The result is then applied to clarify the oftentimes confusing issue
of whether or not the speed of light in non-inertial frames is equal to c.
