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ASPhysicsCIERevision Notes6. Deformation of Solids6.1 Stress & StrainExtension & Compression

Extension & Compression (CIE AS Physics)

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Katie M

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Katie M

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Tensile force

  • Forces can change the size and shape of a body, as well as their motion
    • When forces change the size or shape of a body this is known as deformation
  • Forces acting in opposite directions stretch or compress a body
    • When two forces stretch a body, they are described as tensile
    • When two forces compress a body, they are known as compressive

Tensile and compressive forces

Tensile and compression forces, downloadable AS & A Level Physics revision notes

Tensile forces act to increase the length of an object, compressive forces act to reduce the length of an object in a given dimension

Extension and compression

  • A load attached to the end of a spring
    • applies a force to the spring 
    • which causes it to extend or compress
  • The natural length of a spring is its length without any force applied
  • A spring that increases in length undergoes extension
    • The amount of extension can be calculated by subtracting the natural length from the extended length
  • A spring that decreases in length undergoes compression
    • The amount of compression can be calculated by subtracting the compressed length from the natural length

Tensile force on a spring

Load extension and force, downloadable AS & A Level Physics revision notes

Stretching a spring with a load produces a force that leads to an extension

  • When a spring demonstrates elastic behaviour a load applied creates an extension that can be removed so the spring returns to its original length
    • This occurs until the point at which the spring reaches its limit of proportionality
  • When the spring exceeds the limit of proportionality, the relationship between force and extension is no longer proportional
    • The spring no longer obeys Hooke's law

Force–extension graphs

  • The way a material responds to a given force can be shown on a force-extension graph
    • Every material will have a unique force-extension graph depending on its properties
  • A force-extension graph contains a linear and then a non-linear region
    • The limit of proportionality is the point at which the force-extension graph changes from the linear to the non-linear region

Force-extension graph

Linear and Non-linear Relationship, downloadable IGCSE & GCSE Physics revision notes

Linear and non-linear regions of a force-extension graph

Worked example

Which graph represents the force-extension relationship of a rubber band that is stretched almost to its breaking point?

ANSWER: A

  • Rubber bands can be stretched up to twice their original size or more - this is because the long chain molecules become fully aligned and can no longer move past each other
    • This is shown by graph A - after the section of linear proportionality (the straight line), the gradient increases significantly, so, a large force is required to extend the rubber band by even a small amount
  • Graph B is incorrect as the gradient decreases, suggesting that less force is required to cause a small extension
  • Graph C is incorrect as this shows a material that does not break easily, such as a metal
  • Graph D is incorrect as the plateau suggests no extra force is required to extend the rubber as it has already been stretched

Worked example

Cylindrical samples of steel, glass and rubber are each subjected to a gradually increasing tensile force F. The extensions e are measured and graphs are plotted as shown below. Correctly label the graphs with the materials: steel, glass, rubber.

Worked example - tensile force graphs(2), downloadable AS & A Level Physics revision notes

Examiner Tip

Exam questions may ask for the total length of a material after a load is placed on it and it has extended. Remember to add the extension to the original length of the material to get its final full length

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    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.