Modelling assumptions

Particle

• No dimensions - negligible

Model:

• Mass of object - concentrated at a single point

• Ignore rotational forces + air resistance

Rod

• All dimensions but one are negligible - pole or beam

Modelling:

• mass concentrated along single line - but model at single point

• No thickness

• Rigid - does not bend or buckle

Lamina

• Object with area but negligible thickness - e.g. piece of paper

Modelling assumptions

• mass is distributed across a flat surface

Uniform body

• Mass is distributed evenly

Modelling assumption:

• Mass of 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 that of other objects, e.g. a string or pulley

Modelling:

• Treat object as having 0 mass

• Tension same at both ends of a light string

Inextensible string

• A string that doesn’t stretch under load

Modelling:

• 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 - if not marked as smooth

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

Wire

Rigid, thin length of metal

Modelling - treated as one-dimensional

Smooth and light pulley

All pulleys will be smooth and light in this

Modelling:

• 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:

• Moves freely along a wire or string

• Tension is the same on either side of bead

Peg

• A support from which a body can be suspended or rested

Modelling:

• Dimensionless and fixed

• Can be rough or smooth as specified in question

Air resistance

• Resistance experienced as an object moves through the air

Modelling: usually modelled as being negligible

Gravity

• Force of attraction between all objects - acceleration due to gravity is notated as g

Modelling:

• Assume that all objects with mass are attracted towards the earth

• Earth’s gravity is uniform and acts vertically downwards

• g is constant and taken as 9.8m/s² unless stated other otherwise