modelling assumption in mechanics

particle

dimensions of the object are negligible

modelling assumptions of a particle

mass of the object is concentrated at a single point

rotational forces and air resistance can be ignored

rod

all dimensions but one are negligible, like a pole or a beam

modelling assumptions for a rod

mass is concentrated along a line

no thickness

rigid (does not bend or buckle)

lamina

object with area but negligible thickness, like a sheet of paper

modelling assumptions of lamina

mass is distributed across a flat surface

uniform body

mass is distributed evenly

modelling assumptions for a uniform body

mass of the object is concentrated at a single point at the geometrical centre of the body - the centre of mass

light object

mass of the object is small compared to other masses, like a string or pulley

modelling assumptions for a light object

treat object as having zero mass

tension the same at both ends of a light string

inextensible string

a string that does not stretch under load

modelling assumption for inextensible strings

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

modelling assumption for a smooth surface

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

modelling assumption for a rough surface

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

modelling assumption for wire

treated as one dimensional

smooth and light pulley

all pulleys you consider will be smooth and light

modelling assumption for a 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 for threading on a wire or string

modelling assumption for a bead

moves freely along a wire or string

tension is the same on either side of the bead

peg

a support from which a body can be suspended or rested

modelling assumptions for pegs

dimensionless and fixed

can be rough or smooth as specified in question

air resistance

resistance experienced as an object moves through the air

modelling assumptions for air resistance

usually modelled as being negligible

gravity

force of attracting between all objects

modelling assumption for gravity

assume that all objects with mass are attracted towards the Earth

Earth’s gravity is uniform and acts vertically downwards