modelling assumptions

particle

dimensions are negligible

mass is concentrated at a single point

ignore rotational forces and air resistance

rod

all dimensions but one are negligible

mass concentrated along the line

no thickness

rigid (doesn’t buckle or bend)

lamina

object with area but negligible thickness

mass distributed across a flat surface

uniform body

mass distributed evenly

mass of object concentrated at geometric centre of body - centre of mass

light object

mass of object is small

treat object as having zero mass

tension is the same at both ends of a light string

inextensible string

doesn’t stretch under load

acceleration is the same in objects connected by inextensible string

smooth surface

no friction between surface and objects

rough surface

objects in contact with surface experience frictional forces if in motion or acted on by a force

wire

rigid thin length of metal

one dimensional

smooth and light pulley

pulley has no mass

tension is the same on either side of the pulley

bead

particle with a hole in it

moves freely along a wire or string

tension is the same on either side of the bead

peg

a support on which a body is suspended or rested

dimensionless and fixed

can be rough or smooth

air resistance

usually modelled as being negligible

gravity

force of attraction between all objects

assume all objects with mass are attracted towards Earth

Earth’s gravity is uniform and acts vertically downwards

g is constant at 9.8 ms^-2 unless otherwise stated