Following the classical dynamics of Newton and Euler, the motion of a material body is produced by the action of externally applied forces which are assumed to be of two kinds: surface forces F {\displaystyle \mathbf {F} } and body forces b {\displaystyle \mathbf {b} } .[5] Thus, the total force F {\displaystyle {\mathcal {F}}} applied to a body or to a portion of the body can be expressed as: