Mechanics assumptions

Particle - DImensions of the object are negligible

Particle - DImensions of the object are negligible

• 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 beam

• mass is concentrated along a line

• no thickness

• rigis (does not bend or buckle)

Lamina - Object with area but negligible thickness, like a sheet of paper

• mass is distributed across a flat surface

Uniform body - Mass is distributed evenly

• 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 a pulley

• 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

• 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

Wire - Rigid thin length of metal

• treated as one-dimensional

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 - Particle with a hole in it for threading on a wire or string

• 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

• dimensionless and fixed

• can be rough or smooth as specified in question

Air resistance - Resistance experienced as an object moves through the air

• usually modelled as bring negligible

Gravity - Force of attraction between all objects. Acceleration due to gravity is donoted by g

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

• Earth’s gravity is uniform and acts vertically downwards

• g is a constant and is taken as 9.8 m s^-2 unless otherwise stated in the question