English: See Strain_tensor_field_for_bending.svg. If the strain 1+ e is too close to 1, then an icon for tensors with negative eigenvalues can be used. This one is inspired by the tensor for two springs, with the understanding the a negative spring constant means instability. The images plot a single contour of potential energy, which is ${\displaystyle {\frac {1}{2}}k(x^{2}+y^{2})=U}$ for two identical springs oriented at right angles. Red shading indicates regions of high potential energy, so in (a) the particle is in a potential well, attracted to the center. Unstable equilibrium at the center is depicted in (b), where the the center is like the top of a hill. The saddle points are (c) and (d). In the former, the objects on the vertical axis are pushed away from the red and towards the center, from there they are likely to fall either to the right or the left. In (d) the ultimate fate of almost all particles is to fall upwards or downwards in vertical directions.