Modelling

Question 1

Particle assumption

Answer 1

• mass of the object is concentrated at a single point

- rotational forces and air resistance can be ignored

Question 2

Rod assumption

Answer 2

• mass is concentrated along a line
• no thickness
• rigid (does not bend or buckle)

Question 3

Lamina assumption

Answer 3

• mass is distributed across a flat surface

Question 4

Uniform Body assumption

Answer 4

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

Question 5

Light Object assumption

Answer 5

• treat object as having zero mass

- tension the same at both ends of a light string

Question 6

Inextensible String assumption

Answer 6

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

Question 7

Smooth Surface assumption

Answer 7

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

Question 8

Rough Surface assumption

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 assumption

Answer 9

• treated as one dimensional

Question 10

Smooth and Light Pulley

Answer 10

• pulley as no mass

- tension is the same on either side of the pulley

Question 11

Bead assumption

Answer 11

• moves freely along a wire or string

- tension is the same on either side of the bead

Question 12

Peg assumption

Answer 12

• dimensions are fixed

- can be rough or smooth as specified in the question

Question 13

Air Resistance assumption

Answer 13

• usually modelled as being negligible

Question 14

Gravity assumption

Answer 14

• 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 ms-1, unless otherwise stated in the question