(B) describes the

volume elasticity of a material. Suppose that a

uniformly distributed force acts on the

surface on an

object and is directed

perpendicular to the surface at all points. Then if

*F* is the force acting on and perpendicular to an area

*A*, then the pressure on

**A** can be defined as:

*Pressure on ***A** = **P** = F / **A**

The SI unit for pressure is **Pa**.

Suppose that the pressure on an object of original volume **V**_{0} is increased by an amount ΔV, where ΔV will be negative. It is then defined:

*Volume stress* = Δ**P** *Volume Strain* = Δ**V** / **V**_{0}

Then...

Bulk Modulus = stress / strain

**B** = - Δ**P** / [ Δ**V** / **V**_{0} ] = - **V**_{0}Δ**P** / Δ**V**

The minus sign is used so as to cancel the negative numerical value of ΔV and thereby make **B** a positive number. The bulk modulus has the units of pressure. The reciprocal of the bulk modulus i called the *compressibility K* of the substance.