In classical physics one needs to know the velocity or momentum of a particle in order to determine its evolution. In quantum mechanics the wave function ψ already contains information of possible momenta. What one has to do is to apply harmonic analysis to the function ψ. Sometimes it is helpful to describe quantum states in terms of wave function of momentum. This requires decomposition of ψ in terms of various momentum states and constructing a new function C(k). The value of C(k) of each k gives the strength of the contribution to the k−momentum state of ψ. The space of k is called momentum space.