Edexcel A Level Physics

Revision Notes

4.6 Stress, Strain & The Young Modulus

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Stress & Strain


  • Stress is the applied force per unit cross sectional area of a material
  • Forces can be;
    • Tensile forces, which pull on an object and extend it
    • Compressive forces, which push onto an object and compress (or squash) it

Tensile and compression forces

  •  The equation for stress is the force per unit area, and so the units are N m−2, or Pascals, the same unit as pressureStress equation, downloadable AS & A Level Physics revision notes

Stress equation


  • The ultimate tensile stress is the maximum force per original cross-sectional area a wire is able to support until it breaks



  • Strain is the extension per unit length


Strain is the ratio of the extension (or compression) and the original length

  • This is a deformation of a solid due to stress in the form of elongation or contraction
  • Note that strain is a dimensionless unit because it’s the ratio of lengths


Strain equation


The Young Modulus

Young Modulus

  • The Young modulus (sometimes called Young's Modulus) is the measure of the ability of a material to withstand changes in length with an added load ie. how stiff a material is
  • This gives information about the elasticity of a material
  • The Young Modulus is defined as the ratio of stress and strain


Young Modulus equation


  • Its unit is the same as stress: Pa (since strain is unitless)
  • Just like the Force-Extension graph, stress and strain are directly proportional to one another for a material exhibiting elastic behaviour

Stress-strain graph, downloadable AS & A Level Physics revision notes

A stress-strain graph is a straight line with its gradient equal to Young modulus


  • The gradient of a stress-strain graph when it is linear is the Young Modulus


Worked example

A metal wire that is supported vertically from a fixed point has a load of 92 N applied to the lower end.

The wire has a cross-sectional area of 0.04 mm2 and obeys Hooke’s law.

The length of the wire increases by 0.50%.What is the Young modulus of the metal wire?

A.    4.6 × 107Pa         B.    4.6 × 1012 Pa             

C.    4.6 × 109 Pa         D.    4.6 × 1011 Pa


Exam Tip

To remember whether stress or strain comes first in the Young modulus equation, try thinking of the phrase ‘When you’re stressed, you show the strain’ i.e. Stress ÷ strain.

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Lindsay Gilmour

Author: Lindsay Gilmour

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.