6.1.1 Extension & Compression

• 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

• 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”

ANSWER: A

• 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