What is defined as the ratio of an object's change in length to its original length under applied force?

The concept being described is known as strain, which is defined as the measure of deformation representing the displacement between particles in a material body. Strain is expressed as the ratio of the change in length (deformation) to the original length of the object when a force is applied. It quantifies how much a material deforms under stress.

In practical terms, if you were to stretch a rubber band, the strain would tell you how much longer the rubber band becomes compared to its original length. This ratio is dimensionless, meaning it does not have units because it represents a proportion.

Tension refers to the force that is applied to an object, which may result in strain but is not the ratio itself. Stress, on the other hand, measures the internal forces within a material (force per unit area) but is distinct from strain, which focuses solely on the deformation of the material itself. Shear relates to the type of deformation that occurs when forces are applied parallel or tangential to one of a material's surfaces, but again is not a measure of length change like strain.

Understanding the definition of strain is fundamental in civil engineering, as it helps engineers assess how structures will behave under various loads and conditions.