Strain
The fractional change in dimension due to applied stress: ε = Δx/x, where Δx is change in length and x is original length. A dimensionless quantity. Strain measures deformation in response to stress. Below the elastic limit, strain is reversible; beyond it, permanent deformation (plastic strain) occurs.
Formula
ε = Δx/x
Real World
The cables on the Clifton Suspension Bridge stretch by a few centimetres under heavy traffic — a tiny strain (~10⁻⁴) that keeps the structure within safe elastic limits.
Exam Focus
Strain is dimensionless — never give it units; if asked to 'show your working', write the ratio Δx/x explicitly.
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