Which of the following is not a form of potential energy?

Gravitational potential energy: This is the energy stored in an object as a result of its vertical position or height. It is calculated using the formula: $U=mgh$, where $m$ is mass, $g$ is the acceleration due to gravity, and $h$ is height.

Elastic potential energy: This is the energy stored in elastic materials as the result of their stretching or compressing. It is given by the formula: , where $k$ is the spring constant and $x$ is the displacement from the equilibrium position.