  The momentum representation is seldom used in quantum mechanics courses. Some
students are thence surprised by the change in viewpoint when, in doing
advanced work, they have to use the momentum rather than the coordinate
representation. In this work, we give an introduction to quantum mechanics in
momentum space, where the Schr\"odinger equation becomes an integral equation.
To this end we discuss standard problems, namely, the free particle, the
quantum motion under a constant potential, a particle interacting with a
potential step, and the motion of a particle under a harmonic potential. What
is not so standard is that they are all conceived from momentum space and hence
they, with the exception of the free particle, are not equivalent to the
coordinate space ones with the same names. All the problems are solved within
the momentum representation making no reference to the systems they correspond
to in the coordinate representation.
