with "ρp" and "ρf" the mass densities of the sphere and fluid, respectively, and "g" the gravitational acceleration. Requiring the force balance "Fd" = "Fg" and solving for the velocity "v" gives the terminal velocity "vs". Note that since the excess force increases as "R3" and Stokes' drag increases as "R", the terminal velocity increases as "R2" and thus varies greatly with particle size as shown below. If a particle only experiences its own weight while falling in a viscous fluid, then a terminal velocity is reached when the sum of the frictional and the buoyant forces on the particle due to the fluid exactly balances the gravitational force. This velocity "v" (m/s) is given by: