strain, first consider a two-dimensional infinitesimal material element around a material point (Figure 4), with a unit area in the direction parallel to the - plane, i.e., perpendicular to the page or screen. From equilibrium of forces on the infinitesimal element, the magnitudes of the normal stress and the shear stress are given by: {| class="toccolours collapsible collapsed" width="60%" style="text-align:left" !Derivation of Mohr's circle parametric equations - Equilibrium of forces |- |From equilibrium of forces in the direction of (-axis) (Figure 4), and knowing that the area of the plane where acts is , we have: However, knowing that we obtain Now, from equilibrium of forces in the direction of (-axis) (Figure 4), and knowing that the area of the plane where acts is , we have: However, knowing that we obtain |} Both equations can also be obtained by applying the tensor transformation law on the known Cauchy stress tensor, which is equivalent to performing the static equilibrium of forces in the direction of and . {| class="toccolours collapsible collapsed" width="60%" style="text-align:left" !Derivation of Mohr's circle parametric equations - Tensor transformation |- |The stress tensor transformation law can be stated as Expanding the right hand side, and knowing that and , we have: However, knowing that we obtain However, knowing that we obtain It is not necessary at this moment to calculate the stress component acting on the plane perpendicular to the plane of action of as it is not required for deriving the equation for the Mohr circle. |} These two equations are the parametric equations of the Mohr circle. In these equations, is the parameter, and and are the coordinates. This means that by choosing a coordinate system with abscissa and ordinate , giving values to the parameter will place the points obtained lying on a circle. Eliminating the parameter from these parametric equations will yield the non-parametric equation of the Mohr circle. This can be achieved by rearranging the equations for and , first transposing the first term in the first equation and squaring both sides of each of the equations then adding them. Thus we have where This is the equation of a circle (the Mohr circle) of the form with radius centered at a point with coordinates in the coordinate system. Sign conventions There are two separate sets of sign conventions that need to be considered when using the Mohr Circle: One sign convention