Modelling Assumptions

gravity

assume all objects with mass experience gravity

uniform and acts vertically down

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

inextensible string

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

particle

mass is concentrated at a single point

rotational forces and air resistance are ignored

rod

mass concentrated along a line

no thickness

rigid

lamina

mass distributed across a flat surface

uniform body

mass concentrated at a single point (centre of mass)

light object

treat as having 0 mass

tension the same at both ends of a light string

smooth surface

no friction

rough surface

experience a frictional force if acted on by a force

wire

treated as one dimensional

smooth and light pulley

pulley has no mass

tension the same on each side

bead

moves freely on a wire/string

tension the same each side of the bead

peg

dimensionless and fixed

can be rough or smooth

air resistance

usually modelled as negligible