Assumtions For Modelling Mechanics

Question 1

Particle

Answer 1

• Rotational force and air resistance can be ignored

- mass of object is concentrated at a single point

Question 2

Rod

Answer 2

• mass is concentrated along a line
• no thickness
• rigid

Question 3

Lamina

Answer 3

-mass is distributed along a flat surface

Question 4

Uniform body

Answer 4

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

Question 5

Light object

Answer 5

• treat object as having zero mass

- tension the same at both ends of the light string

Question 6

Inextensible string

Answer 6

-accelerating is the same in object connected by a taut inextensible string

Question 7

Smooth surface

Answer 7

-assume that there is no friction between the surface and the object

Question 8

Rough surface

Answer 8

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

Question 9

Wire

Answer 9

-treated as one dimensional

Question 10

Smooth and light pulley

Answer 10

• pulley has no mass

- tension is the same on either side of the pulley

Question 11

Bead

Answer 11

• moves freely along a wire or string

- tension is the same on either side of the bead

Question 12

Peg

Answer 12

• dimensionless and fixed

- can be rough or smooth as specific in question

Question 13

Air resistance

Answer 13

-usually modelled as being negligible

Question 14

Gravity

Answer 14

• assume that all objects with mass are attracted towards the earth
• earths gravity is uniform and facts vertically downwards
• g is a constant and is tasked as 9.8ms-1 unless otherwise stated in the question.