In quantum mechanics, the momentum operator is the operator associated with the linear momentum. The momentum operator is, in the position representation, an example of a differential operator. For the case of one particle in one spatial dimension, the definition is: p ^ ^ = − − i ℏ ℏ ∂ ∂ ∂ ∂ x where ħ is the reduced Planck constant, i the imaginary unit, x is the spatial coordinate, and a partial derivative (denoted by ∂ ∂ / ∂ ∂ x ) is used instead of a total derivative (d / dx) since the wave function is also a function of time. The "hat" indicates an operator. The "application" of the operator on a differentiable wave function is as follows: p ^ ^ ψ ψ = − − i ℏ ℏ ∂ ∂ ψ ψ ∂ ∂ x