### 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**

- When two 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**

#### 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**. This is explored further in the revision notes “The Spring Constant”*k*

- 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