Let  be the normal stress in the liquid that points radially outward from the center of the bubble.  In spherical coordinates, for a fluid with constant density and constant viscosity,
Therefore at some small portion of the bubble surface, the net force per unit area acting on the lamina is
where  is the surface tension.  If there is no mass transfer across the boundary, then this force per unit area must be zero, therefore
and so the result from momentum conservation becomes
whereby rearranging and letting  gives the Rayleigh–Plesset equation
Using dot notation to represent derivatives with respect to time, the Rayleigh–Plesset equation can be more succinctly written as