In the design process of any device, the computation of the forces or the torque is often a decisive step. Finite element solutions give accurate solutions in terms of magnetic field or flux density. But for local force computation associated to a magnetic field, many formulations are available. They use different physical interpretations of the same phenomenon. Here, a new experiment is presented. A test body is submitted to an external field and its deformation is observed. With an unusual material, the effects of external strains induced by magnetic or electric fields are increased. Numerical simulations allow the calculation of local forces associated to all formulations. Consequently it is possible to use them to deduce the body deformations. It appears that the body deformation associated to the local force provided by the energy principle corresponds to the experiment