  An analytical model for adding a space dependent potential to the phi^4 field
is presented, by constructing a collective coordinate for solitary solution of
this model. Interaction of the $\phi^{4}$ field with a delta function potential
barrier and also delta function potential well is investigated. Most of the
characters of the interaction are derived analytically while they are
calculated by other models numerically. We will find that the behaviour of the
solitary solution is like a point particle 'living' under the influence of a
complicated potential which is a function of the field initial conditions and
the potential parameters.
