Mechanics Chapter 8 Modelling Assumptions

Question 1

Particle

Answer 1

• objects dimensions are negligible
• mass concentrated at one point
• rotational forces and air resistance ignored

Question 2

Rod

Answer 2

• all dimensions bur length is negligible (so no thickness)
• mass concentrated along a line
• rigid, no bending

Question 3

Lamina

Answer 3

• object with area not negligible thickness (e.g. paper)

- mass distributed across a flat surface

Question 4

Uniform body

Answer 4

-mass is distributed unevenly and concentrated at the geometrical centre of the body (the centre of mass)

Question 5

Light object

Answer 5

• mass of object is small compared to other masses (like a string)
• treated as zero mass
• tension is the same at either end of object

Question 6

Inextensible string

Answer 6

• string that doesn’t stretch under load

- acceleration in all objects connected to the string is the same

Question 7

Smooth surface

Answer 7

-there is no friction between the surface and any objects on it

Question 8

Rough surface

Answer 8

-objects in contact with the surface experience frictional forces when moving

Question 9

Wire

Answer 9

• rigid thin length of metal

- one dimensional

Question 10

pulley

Answer 10

• all pulleys are smooth and light
• no mass
• tension equal all over

Question 11

Bead

Answer 11

• Particle with a hole in it for threading wire or string
• moves freely on wire or string
• tension equal everywhere

Question 12

Peg

Answer 12

• support from which a body can be suspended from
• dimensionless
• can be rough or smooth

Question 13

Air resistance

Answer 13

• resistance experienced as an object moves through the air

- usually modelled as negligible

Question 14

Gravity

Answer 14

• force of attraction between all objects
• assume all objects are attracted to earth
• uniform and acts vertically down
• acceleration due to gravity = g = 9.8 m/s