tetrahedron of material of Fig 3. The area of the faces of the tetrahedron are △S1,△S2,△S3and△S. The stress vectors on planes with reversed normals t(ei have been replaced with -t() using Newton's third law of action and reaction(which is in fact derivable from equilibrium): t(-n)=-t(n Enforcing equilibrium we have t)△S-t)△S1-t2)△S2-t(△s3=0 (3) n) Figure 3: Cauchy 's tetrahedron representing the equilibrium of a tetrahedron shrinking to a point where AV is the volume of the tetrahedron and f is the body force per unit volume. The following relation: ASni= ASi derived in the following mathematical aside y virtue of Green's Theorem 3� � tetrahedron of material of Fig.3. The area of the faces of the tetrahedron are ΔS1, ΔS2, ΔS3 and ΔS. The stress vectors on planes with reversed normals t(−ei) have been replaced with −t(i) using Newton’s third law of action and reaction (which is in fact derivable from equilibrium): t(−n) = −t(n) . Enforcing equilibrium we have: t(n) ΔS − t(1)ΔS1 − t(2)ΔS2 − t(3)ΔS3 = 0 (3) t (3) − t (2) − t (1) − −e −e −e 1 2 3 n t(n) Figure 3: Cauchy’s tetrahedron representing the equilibrium of a tetrahedron shrinking to a point where ΔV is the volume of the tetrahedron and f is the body force per unit volume. The following relation: ΔSni = ΔSi derived in the following mathematical aside: By virtue of Green’s Theorem: �φdV = nφdS V S 3