Mechanics 8.2 Modelling assumptions

Question 1

Model of a Particle

Answer 1

Particle - Dimensions of the object are negligible

Question 2

Rod Model

Answer 2

Rod - All dimensions but one are negligible, like a pole or a beam

Question 3

Lamina model

Answer 3

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

Question 4

Uniform body model

Answer 4

Uniform body -mass is distributed evenly

Question 5

Light object model

Answer 5

Light object - Mass of the object is small compared to other masses, like a string or pulley

Question 6

Inextensible string model

Answer 6

Inextensible string - A string that does not stretch under load

Question 7

Rough surface model

Answer 7

Rough surface - If a surface is not smooth, it is rough

Question 8

Wire model

Answer 8

Wire - rigid thin length of metal

Question 9

Smooth and light pulley model

Answer 9

Smooth and light pulley - all pulleys you consider will be smooth and light

Question 10

Bead model

Answer 10

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

Question 11

Peg model

Answer 11

Peg - A support from which a body can be suspended or rested

Question 12

Air resistance model

Answer 12

Resistance experienced as an object moves through the air

Question 13

Gravity model

Answer 13

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

Question 14

Modelling assumptions for a particle

Answer 14

-Mass of all the object is concentrated at a single point
-Rotational forces and air resistance can be ignored

Question 15

Modelling assumptions for a rod

Answer 15

-Mass is concentrated along a line
-No thickness
-Rigid (doesn’t bend or buckle)

Question 16

Modelling assumptions for a Lamina

Answer 16

-Mass is distributed across a flat surface

Question 17

Modelling assumptions for a Uniform Body

Answer 17

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

Question 18

Modelling assumptions for a Light object

Answer 18

-Treat object as having zero mass
-Tension the same at both ends of a light string

Question 19

Modelling assumptions for an Inextensible string

Answer 19

-Acceleration is the same in objects connected by a taut Inextensible string

Question 20

Modelling assumptions for Smooth surface

Answer 20

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

Question 21

Modelling assumptions for a rough surface

Answer 21

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

Question 22

Modelling assumptions for a Wire

Answer 22

-Treated as one dimensional

Question 23

Modelling assumptions for a smooth and light pulley

Answer 23

-Pulley has no mass
-tension is the same on either side of the bead

Question 24

Modelling assumptions for a Peg

Answer 24

-Dimensionless and fixed
-Can be rough or smooth as specified in question

Question 25

Modelling assumptions for Air resistance

Answer 25

-Usually modelled as being negligible

Question 26

Modelling assumptions for Gravity

Answer 26

-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.