modelling assumptions

particle

• mass of the object is concentrated at a single point

• rotational forces and air resistances can be ignored

rod

• mass is concentrated along a line

• no thickness

• rigid (does not bend or buckle)

lamina

• mass is distributed across a flat surface

uniform body

• mass of the object is concentrated at a single point at the geometrical centre of the body (centre of mass)

light object

• treat object as having zero mass

  • tension the same at both ends of a light string

inextensible string

• acceleration is the same in objects connected by a taut inextensible string

smooth surface

• assume that there is no friction between the surface and any object on it

rough surface

• objects in contact with the surface experience a frictional force if they are being acted on by a force or moving

wire

• treated as one dimensional

smooth and light pulley

• pulley has no mass

• tension is the same on either side of the pulley

bead

• moves freely along a wire or string

• tension is the same on either side of the bead

peg

• dimensionless and fixed

• can be rough or smooth

air resistance

• usually modelled as being negligible

gravity

• assume that all objects with mass are attracted towards the Earth

• Earth’s gravity is uniform and acts vertically down

• g is constant and is taken as 9.8ms^-2 unless stated otherwise