• Body Force: distributed over the entire mass or volume of the element. It is usually expressed per unit mass of the element or medium upon which the forces act.
  Example: Gravitational Force, Electromagnetic force fields etc.
  
• Surface Force: Forces exerted on the fluid element by its surroundings through direct contact at the surface.
      Surface force has two components:
  • Normal Force: along the normal to the area
  • Shear Force: along the plane of the area.

The ratios of these forces and the elemental area in the limit of the area tending to zero are called the normal and shear stresses respectively.

The shear force is zero for any fluid element at rest and hence the only surface force on a fluid element is the normal component.