  In quantum mechanics textbooks the momentum operator is defined in the
Cartesian coordinates and rarely the form of the momentum operator in spherical
polar coordinates is discussed. Consequently one always generalizes the
Cartesian prescription to other coordinates and falls in a trap. In this work
we introduce the difficulties one faces when the question of the momentum
operator in general curvilinear coordinates arises. We have tried to elucidate
the points related to the definition of the momentum operator taking spherical
polar coordinates as our specimen coordinate system and proposed an elementary
method in which we can ascertain the form of the momentum operator in general
coordinate systems.
