When there is no flow through the packed bed, the net gravitational force (including bouyancy) acts downward. When flow begins upward, friction forces act upward and counterbalance the net gravitational force. For a high enough fluid velocity, the friction force is large enough to lift the particles. This represents the onset of fluidization. The frictional force can be expressed in terms of a friction factor. This leads to equations describing the the flow of a fluid past a collection of particles. The Ergun equation is one such equation.