Extension & Compression (CIE A Level Physics)

• Forces can change the size and shape of a body, as well as their motion
  • When forces change the size or shape of a body this is known as deformation
• Forces acting 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 compressive forces

Tensile forces act to increase the length of an object, compressive forces act to reduce the length of an object in a given dimension

• A load attached to the end of a spring
  • applies a force to the spring 
  • which causes it to extend or compress
• The natural length of a spring is its length without any force applied
• A spring that increases in length undergoes extension
  • The amount of extension can be calculated by subtracting the natural length from the extended length
• A spring that decreases in length undergoes compression
  • The amount of compression can be calculated by subtracting the compressed length from the natural length

Tensile force on a spring

Stretching a spring with a load produces a force that leads to an extension

• When a spring demonstrates elastic behaviour a load applied creates an extension that can be removed so the spring returns to its original length
  • This occurs until the point at which the spring reaches its limit of proportionality
• When the spring exceeds the limit of proportionality, the relationship between force and extension is no longer proportional
  • The spring no longer obeys Hooke's law

Force–extension graphs

• The way a material responds to a given force can be shown on a force-extension graph
  • Every material will have a unique force-extension graph depending on its properties
• A force-extension graph contains a linear and then a non-linear region
  • The limit of proportionality is the point at which the force-extension graph changes from the linear to the non-linear region

Force-extension graph

Linear and non-linear regions of a force-extension graph