# 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

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

#### 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.

#### 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

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 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

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

• 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 it has 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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