Objects are not rigid, which are deformed by tension, compression, pressure, or shear.

conditions for equilibrium:

vector sum of all external forces and torques must be zero.

Relationship between force F, torque τ (or moment M), linear momentum p, and angular momentum L:

$vec tau (vec M)= vec r times vec F$

$vec L = vec r times vec p$

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Tensile(Compressive) stress = $frac{F}{A}$

Tensile(Compressive) strain = $frac{Delta l}{l_0}$

Young’s modulus: E = Tensile stress / Tensile strain

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Shear stress = $frac{F_{||}}{A}$

Shear strain = $frac x h$

Shear modulus: G = Shear stress / Shear strain

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Bulk stress = $ Delta p$

Bulk strain = $ frac{Delta V}{V_0}$

Bulk modulus: K = $-frac{Delta p}{Delta V/V_0}$

Compressibility: 1/K