In mechanics, it is necessary to take a real life problem and put it in mathematical language. This process is known as **modelling**.

**Terminology**

When modelling, we often make assumptions to make the mathematics simpler.

A **particle** is a body whose entire weight acts through a single point. A particle doesn’t suffer from air resistance.

A **lamina** is a two dimensional body, it is the 2-dimensional equivalent of a particle. All of the weight acts through one plane.

A body is said to be **uniform** if it has constant density. A body is non-uniform if the density varies throughout.

A **smooth** surface is one which has no friction.

A **rough** surface is one upon which friction acts.

We say an object is **light** if it has no mass. So no weight acts on a light body.

An **inextensible string** is a string which has a fixed length- it is impossible to stretch.

A **rigid body** is a body which is not a point and whose shape is fixed.