Linear Momentum
In general, there are two types of forces acting on the fluids: surface forces and body forces. Surface forces (Fs ) are external forces acting on the surfaces between solids and the fluids or between different fluid layers. Examples: pressure forces, shear forces, surface tension, etc. Body forces (FB)are acting on the entire body of the fluids, such as gravity and magnetohydrodynamic forces.
Note: These forces are forces acting on the fluids in order to produce the necessary changes of the linear momentum of the fluids. In order to evaluate the forces acting on the external devices, one needs to apply the Newton’s third law: reaction force is equal in magnitude and opposite in direction with respect to the action force. That is, R = - F