8.2 Modelling Assumptions

Question 1

Particle

Answer 1

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

Question 2

Rod

Answer 2

• Mass is concentrated along a line
• No thickness
• rigid (does not bend or buckle)

Question 3

Lamina

Answer 3

• Mass is distributed across a flat surface

Question 4

Uniform body

Answer 4

• Mass of the 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 as both ends of a light string

Question 6

Inextensible string

Answer 6

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

Question 7

Smooth Surface

Answer 7

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

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 specified 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
• Earth’s gravity is uniform and acts vertically downwards
• Gravity is constant and is taken as 9.8ms^-2, unless otherwise stated in the question