# Illustration Classically forbidden / allowed region

Harmonic potential energy function $$W_{\text{pot}}(x)$$ with sketched total energy $$W$$ of a particle. Classically the particle always has a positive kinetic energy:$$W_{\text{kin}} ~=~ (W_{\text{pot}} - W) > 0$$Here the particle can only move between the turning points $$x_1$$ and $$x_2$$, which are determined by the total energy $$W$$ (horizontal line). It is the classically allowed region (blue).
Negative kinetic energy is not physically possible:$$W_{\text{kin}} ~=~ (W_{\text{pot}} - W) < 0$$