modelling assumptions

particle

• mass of object concentrated at a single point

• rotational forces and air resistance can be ignored

rod

• mass concentrated along a line

• no thickness

• rigid

lamina

• mass distributed across a flat surface

• negligible thickness

uniform body

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

light object

• treat object as having zero mass

• tension the same at both ends of a light string

inextensible string

• does not stretch under load

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

smooth surface

no friction

rough surface

there is friction

wire

treated as 1d

smooth and light pulley

• pulley has no mass

• tension is the same on either side of the pulley

bead (particle w hole in it for threading on a wire/ string)

• moves freely along a wire/ string

• tension is the same on other side of bead

peg

dimensionless and fixed

air resistance

negligible

gravity

• assume all objects w mass are attracted towards the earth

• earths gravity is uniform and acts vertically downwards

• g is constant and taken as 9.8