text: The equation of motion for the superfluid component, in a somewhat simplified form, is given by Newton's law
  The mass M is the molar mass of He, and  is the velocity of the superfluid component. The time derivative is the so-called hydrodynamic derivative, i.e. the rate of increase of the velocity when moving with the fluid. In the case of superfluid He in the gravitational field the force is given by
  In this expression μ is the molar chemical potential, g the gravitational acceleration, and z the vertical coordinate. Thus we get the equation which states that the thermodynamics of a certain constant will be amplified by the force of the natural gravitational acceleration  
  Eq.  only holds if v is below a certain critical value, which usually is determined by the diameter of the flow channel.
  In classical mechanics the force is often the gradient of a potential energy. Eq.  shows that, in the case of the superfluid component, the force contains a term due to the gradient of the chemical potential. This is the origin of the remarkable properties of He-II such as the fountain effect.