Mechanics 8.2 Modelling assumptions Flashcards
Model of a Particle
Particle - Dimensions of the object are negligible
Rod Model
Rod - All dimensions but one are negligible, like a pole or a beam
Lamina model
Lamina - Object with area but negligible thickness, like a sheet of paper
Uniform body model
Uniform body -mass is distributed evenly
Light object model
Light object - Mass of the object is small compared to other masses, like a string or pulley
Inextensible string model
Inextensible string - A string that does not stretch under load
Rough surface model
Rough surface - If a surface is not smooth, it is rough
Wire model
Wire - rigid thin length of metal
Smooth and light pulley model
Smooth and light pulley - all pulleys you consider will be smooth and light
Bead model
Bead - Particle with a hole in it for threading on a wire or string
Peg model
Peg - A support from which a body can be suspended or rested
Air resistance model
Resistance experienced as an object moves through the air
Gravity model
Gravity - Force of attraction between all objects. Acceleration due to gravity is denoted by g.
Modelling assumptions for a particle
-Mass of all the object is concentrated at a single point
-Rotational forces and air resistance can be ignored
Modelling assumptions for a rod
-Mass is concentrated along a line
-No thickness
-Rigid (doesn’t bend or buckle)
Modelling assumptions for a Lamina
-Mass is distributed across a flat surface
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
Modelling assumptions for a Light object
-Treat object as having zero mass
-Tension the same at both ends of a light string
Modelling assumptions for an Inextensible string
-Acceleration is the same in objects connected by a taut Inextensible string
Modelling assumptions for Smooth surface
-Assume that there is no friction between the surface and any object on it
Modelling assumptions 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 assumptions for a Wire
-Treated as one dimensional
Modelling assumptions for a smooth and light pulley
-Pulley has no mass
-tension is the same on either side of the bead
Modelling assumptions for a Peg
-Dimensionless and fixed
-Can be rough or smooth as specified in question
Modelling assumptions for Air resistance
-Usually modelled as being negligible
Modelling assumptions for Gravity
-Assume that all the 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 s^-2, unless otherwise stated in the question.