External forces include the active forces applied on the structure. For example the pressure force we apply over it and the self-weight of the structure.nInternal forces include the reactive force that is generated inside the structure that prevents the deformation of the structure and holds it stiff. These include the stress developed, the inertia forces, the product of stiffness the small displacements happening in the nodes.nOn the whole, the force which we apply externally must balance all the parameters that are developed internally when converted to equal units for calculation.nHope it helps n