INNER FORCES

There are always certain interaction forces between neighboring particles of the body (molecules, crystals, atoms), otherwise internal forces.
These forces seek to preserve the body as a whole. External forces, on the contrary, always tend to cause deformation of the body.
Internal forces act in the absence of external load, but in this case they are mutually balanced and do not manifest themselves in any way. The ability to assess their level and feel their presence appears only when external forces are applied to the body.
The application to the body of an external load causes a change in internal forces, i.e. the emergence of additional internal forces. In the resistance of materials, only those additional internal forces that appear as a result of loading are studied and calculated. Thus, it becomes necessary to link and express internal efforts through external ones.