  It is shown that the extra coordinate of 5D induced-matter and membrane
theory is related in certain gauges to the inertial rest mass of a test
particle. This implies that the Weak Equivalence Principle is a geometric
symmetry, valid only in the limit in which the test mass is negligible compared
to the source mass. Exact solutions illustrate this, and show the way to
possible resolutions of the cosmological-constant and hierarchy problems.
