Consider two waves passing through the same string. Principle of superposition for displacement says that the net displacements of a particle on the string is sum of the displacements produced by the two waves individually. Suppose we state similar principles for the net velocity of the particle and the net kinetic energy of the particle. Such a principle will be valid for
The principle of superposition is valid only for the vector quantities. Velocity is a vector quantity, whereas the kinetic energy is not.