CIE AS Physics (9702) exams from 2022

Revision Notes

6.1.1 Extension & Compression

Tensile Force

  • Forces don’t just change the motion of a body, but can change the size and shape of them too. This is known as deformation
  • Forces 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 compression forces, downloadable AS & A Level Physics revision notes

Diagram of tensile and compressive forces

 

Tensile Strength

  • Tensile strength is the amount of load or stress a material can handle until it stretches and breaks
  • Here are some common materials and their tensile strength:

 

Tensile strength of various materials

Table of the tensile strength of various materials, downloadable AS & A Level Physics revision notes

 

Worked example

Worked example - tensile force graphs(1), downloadable AS & A Level Physics revision notes

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

Exam Tip

Remember to read the questions carefully in order to not confuse the terms ‘tensile stress’ and ‘tensile strain’.

Extension and Compression

  • When you apply a force (load) onto a spring, it produces a tensile force and causes the spring to extend

 

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

 

Hooke’s Law

  • If a material responds to tensile forces in a way in which the extension produced is proportional to the applied force (load), we say it obeys Hooke’s Law
  • This relationship between force and extension is shown in the graph below

 

Force-extension graph, downloadable AS & A Level Physics revision notes

Force v extension graph for a spring

 

  • The extension of the spring is determined by how much it has increased in length
  • The limit of proportionality is the point beyond which Hooke’s law is no longer true when stretching a material i.e. the extension is no longer proportional to the applied load
    • The point is identified on the graph where the line is no longer straight and starts to curve (flattens out)
  • Hooke’s law also applies to compression as well as extension. The only difference is that an applied force is now proportional to the decrease in length
  • The gradient of this graph is equal to the spring constant k. This is explored further in the revision notes “The Spring Constant”

 

Worked example

Worked example - force extension graphs, downloadable AS & A Level Physics revision notes

  • Rubber bands obey Hooke’s law until they’re 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 which obeys Hooke’s Law and 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 been stretched

 

Exam Tip

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

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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