Mechanics Year 1- Chapter 8: Modelling in mechanics

Question 1

What is a particle?

Answer 1

Dimensions of the object are negligible

Question 2

What are the assumptions for a particle?

Answer 2

Mass of the object is concentrated at a single point

Rotational forces and air resistance can be ignored

Question 3

What is a rod?

Answer 3

All dimensions but one are negligible like a pole or beam

Question 4

What are the assumptions for a rod?

Answer 4

Mass is concentrated along a line

No thickness

Rigid (does not bend or buckle)

Question 5

What is a lamina?

Answer 5

Object with area but a negligible thickness like a sheet of paper

Question 6

What are the assumptions for a lamina?

Answer 6

Mass is distributed across a flat surface

Question 7

What is a uniform body?

Answer 7

Mass is distributed evenly

Question 8

What are the assumptions for a uniform body?

Answer 8

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

Question 9

What is a light object?

Answer 9

Mass of the object is small compared to other masses, like a string or a pulley

Question 10

What are the assumptions for a light object?

Answer 10

Treat object as having zero mass
Tension the same at both ends of a light string

Question 11

What is an inextensible string?

Answer 11

A string that does not stretch under load

Question 12

What are the assumptions for an inextensible spring?

Answer 12

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

Question 13

What are the assumptions for a smooth surface?

Answer 13

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

Question 14

What are the assumptions for a rough surface?

Answer 14

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

Question 15

What is a wire?

Answer 15

Rigid thin length of metal

Question 16

What are the assumptions for a wire?

Answer 16

Treated as one-dimensional

Question 17

What are the assumptions for a smooth and light pulley?

All pulleys you consider will be smooth and light

Answer 17

Pulley has no mass

Tension is the same on either side of the pulley

Question 18

What is a bead?

Answer 18

Particle with a hole in it for threading on a wire or string

Question 19

What are the assumptions for a bead?

Answer 19

Moves freely along a wire or string

Tension is the same on either side of the bead

Question 20

What is a peg?

Answer 20

A support from which a body can be suspended or rested

Question 21

What are the assumptions for a peg?

Answer 21

Dimensionless and fixed
Can be rough or smooth as specified in question

Question 22

What is air resistance?

Answer 22

Resistance experienced as an object moves through the air

Question 23

What are the assumptions for air resistance?

Answer 23

Usually modelled as being negligible

Question 24

What is gravity?

Answer 24

Force of attraction between all objects.

Acceleration due to gravity is denoted by g

Question 25

What are the assumptions for gravity?

Answer 25

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 m s-², unless otherwise stated in the question