For 10 points, name this quantity using units of pressure that measures internal forces for a deformable solid.
One axiom used to define this says that the action on any part of a body is equal to the system of forces on the surface of the body. That axiom can be used to derive the result that two vectors for this acting on opposite sides of a surface have the opposite sign. In addition to the Euler-Cauchy postulate and Cauchy's lemma, this quantity at any point can be represented in the Piola-Kirchoff tensor, which avoids the problem of calculating the area of the body in the deformed configuration that is present in the Cauchy tensor. The (*) yield point gives the value of this quantity past which a material will deform non-reversibly. A two dimensional method of visualizing this quantity plots its normal and shear components in Mohr's circle. The Young's modulus is defined as this quantity over strain. For 10 points, name this quantity using units of pressure that measures internal forces for a deformable solid.
One axiom used to define this says that the action on any part of a body is equal to the system of forces on the surface of the body.
stress [do not prompt on "strain" or "pressure"]