Mechanics - Modelling Assumptions

Question 1

Particle

Answer 1

Dimensions are negligible.
- Mass is concentrated at a single point
- Rotational forces & air resistance can be ignored

Question 2

Rod

Answer 2

All dimensions but one are negligible. (e.g. pole/beam)
- Mass is concentrated along a line
- No thickness
- Rigid (no bend/buckle)

Question 3

Lamina

Answer 3

Area, but negligible thickness (e.g. paper)
- Mass is distributed across flat surface

Question 4

Light Object

Answer 4

Mass is essentially irrelevant (e.g string, pulley)
- Treated as having 0 mass
- Tension is the same at both ends of a light string

Question 5

Uniform Body

Answer 5

Mass is distributed evenly
- Mass is concentrated at geometrical centre of body
—-CENTRE OF MASS—-

Question 6

Inextensible String

Answer 6

Does not stretch under load
- Acceleration is the same when connected by a taut inextensible string

Question 7

Smooth Surface

Answer 7

NO friction between it and objects

Question 8

Rough surface

Answer 8

Anything that is not smooth
- Objects in contact experience friction with movement or when acted upon

Question 9

Wire

Answer 9

Rigid, thin length of metal
- Treated as 1D

Question 10

Smooth & Light Pulley

Answer 10

ALL Pulleys
- No mass
- Tension is same either side

Question 11

Bead

Answer 11

Particle with a hole for wire/string
- Moves freely along wire/string
- Tension is same either side

Question 12

Peg

Answer 12

Support from which body can be suspended/rested
- Dimensionless and fixed
- Can be rough or smooth

Question 13

Air resistance

Answer 13

usually modelled as negligible

Question 14

Gravity

Answer 14

Assume all objects with mass are attracted towards Earth
- Earth’s gravity is uniform and acts vertically downwards
- g (acceleration due to gravity) = 9.9 ms-2 USUALLY