CIE AS Physics (9702) 2019-2021

Revision Notes

7.1.1 Hooke's Law

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’.

Hooke's Law

  • A material obeys Hooke’s Law if its extension is directly proportional to the applied force (load)
  • The Force v Extension graph is a straight line through the origin
  • This linear relationship is represented by the Hooke’s law equation

Hooke's law equation, downloadable AS & A Level Physics revision notes

Hooke’s Law

 

  • The constant of proportionality is known as the spring constant k

Worked example

Worked example - hooke's law (1), downloadable AS & A Level Physics revision notes

Worked example - hooke's law (2), downloadable AS & A Level Physics revision notes

Exam Tip

Double check the axes before finding the spring constant as the gradient of a force-extension graph. Exam questions often swap the load onto the x-axis and length on the y-axis. In this case, the gradient is not the spring constant but 1 ÷ gradient is.

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