  In the one-dimensional Klein-Fock-Gordon theory, the probability density is a
discontinuous function at the point where the step potential is discontinuous.
Thus, the mean value of the external classical force operator cannot be
calculated from the corresponding formula of the mean value. To resolve this
issue, we obtain this quantity directly from the Klein-Fock-Gordon equation in
Hamiltonian form, or the Feshbach-Villars wave equation. Not without surprise,
the result obtained is not proportional to the average of the discontinuity of
the probability density but to the size of the discontinuity. In contrast, in
the one-dimensional Schr\"odinger and Dirac theories this quantity is
proportional to the value that the respective probability density takes at the
point where the step potential is discontinuous. We examine these issues in
detail in this paper. The presentation is suitable for the advanced
undergraduate level.
