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