Assumptions

Model

Modelling assumptions

Particle – Dimensions of the object considered are negligible.

• The mass of the particle is concentrated at a single point.

• Rotational forces and air resistance can be ignored

Rod – Dimensions of the object are negligible.

• mass is concentrated along a line

• the rod has no thickness

• the rod is rigid (cannot be bent)

Lamina – an object with a wide area but negligible thickness, such as a sheet of paper.

• mass is uniformly distributed across a completely flat surface

Uniform body – mass of the body is distributed evenly.

• The mass of the body is concentrated at a single point that is the geometrical center of the body - the center of mass

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

• the object can be treated as having zero mass

• tension is the same at both ends of a light string

Inextensible string – a string that does not stretch under load.

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

Smooth surface – surface with no rugosity, so friction is negligible.

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

Rough surface – if a surface is not a smooth surface, it is considered a rough surface.

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

Wire – rigid thin length of metal.

• treated as one-dimensional object

Smooth and light pulley – all pulleys you consider will be smooth and light.

• pulley has no mass

• tension is the same on either side of the pulley

Bead – a particle with a hole in it for threading through a wire or string.

• moves freely along a wire or string

• tension is uniform on either side of the bead

Peg – support from which a body can be suspended or rested.

• dimensionless and fixed

• can be considered rough or smooth as specified in the question

Air resistance – the resistance experienced by an object as it moves through air.

• can be modeled as being negligible

Gravity – the force of attraction between all objects. Acceleration due to gravity is denoted by g, and g = 9.8 m/s² unless otherwise specified.

• assume that all objects with mass are attracted towards the Earth

• Earth's gravity is uniform and acts vertically downwards

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