# Elastic or Plastic

No material is absolutely rigid. Even a concrete floor changes shape as you walk across it. The behaviour of a material subjected to a tensile (pulling) or compressive (pushing) force can be described as either **elastic** or **plastic**.

- a material is
**elastic**if it returns to its original shape and size when the force is removed - a material is
**plastic**if it does not return to its original shape and size whenthe force is removed

Most materials are elastic for a certain range of forces, up to the **elastic limit**, beyond which they are plastic. Plasticine and playdough are plastic for all forces.

**Hooke** attempted to write a simple rule that describes the behaviour of all materials subjected to a tensile force.

**Hooke’s law states that:the extension of a sample of material is proportional to the stretching force e α FThis can be written as F = kewhere k represents the stiffness of the sample and has units of N m **

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If the extension is proportional to the stretching force, then doubling the force causes the extension to double.

Metals and springs ‘obey’ Hooke’s law up to a certain limit, called the **limit of proportionality**. For small extensions, the extension is proportional to the stretching force. Rubber and other **polymeric** solids do not show this pattern of behaviour.

A polymeric solid is one made up of long chain molecules.

The graphs below contrast the behaviour of different materials subjected to an increasing stretching force.

**Copper**is a**ductile**material, which means that it can be drawn into wires. It is also**malleable**, which means that it can be reshaped by hammering and bending without breaking. When stretched beyond the point E on the graph it retains its new shape.**Rubber**does not follow Hooke’s law and it remains elastic until it breaks.**Glass**is**brittle**; it follows Hooke’s law until it snaps.**Kevlar**is**tough**; it can withstand shock and impact.**Mild steel**is**durable**; it can withstand repeated loading and unloading.**Diamond**is**hard**; it cannot be easily scratched.