It is often important to determine the force produced on a solid body by fluid flowing steadily over or through it. For example, there is the force exerted on a solid surface by a jet of fluid impinging on it; there are also the aerodynamic forces (lift and drag) on an aircraft wing, the force on a pipe-bend caused by the fluid flowing within it, the thrust on a propeller and so on. All these forces are associated with a change in the momentum of the fluid.