English: A picture to illustrate the definition of stress. The force applied by the one infinitesimal particle (top sphere) on an adjacent particle (bottom sphere) across the infinitesimal surface element that separates them (yellow disk) is represented by the large arrow. The stress (represented by the smaller arrows scattered over the disk) is the force divided by the area of the surface element.

The picture used to illustrate the definition of other second-order tensors in 3-space, such as permeability, polarization, etc.

Image created with POV-Ray by J.Stolfi on 2013-02-02.

Source code available at http://www.ic.unicamp.br/~stolfi/misc/2013-02-02-stress-strain-pics.tgz