Hooke's law

Hooke's law states that the force developed in a spring is linearly proportional to the distance travelled.  The law was proposed by Robert Hooke, the 17th century physicist.

Mathematically, it can be expressed as

F = kX

F is the force in newtons
k is the spring constant
X is the distance travelled in metres

Hooke's law for stress and  strain

Hooke's law can be generalized elasticity to say that "the strain of an elastic object is proportional to the stress".  Once the elastic limit is exceeded, Hooke's law will not hold true.

For plastic materials
Strain = Proportionality constant x Stress

The proportionality constant will be unique for different materials.