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(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 V0 is increased by an amount ΔV, where ΔV will be negative. It is then defined:

Volume stress = ΔP Volume Strain = ΔV / V0

Then...

Bulk Modulus = stress / strain

B = - ΔP / [ ΔV / V0 ] = - V0ΔP / ΔV

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.

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