Surface forces and body forces Surface forces: the forces acting on the contact surface between adjacent parts of rock system, between adjacent blocks or adjacent lithosphere plates. The contact surface may be or may be not a visible material boundary It can be a imaginary surface inside the object consideredSurface Forces and Body Forces Surface forces: the forces acting on the contact surface between adjacent parts of rock system, between adjacent blocks or adjacent lithosphere plates. The contact surface may be or may be not a visible material boundary. It can be a imaginary surface inside the object considered