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