Chapter 8 - Modelling in Mechanics

Question 1

Particle:

Answer 1

Dimensions of the object are negligible

Question 2

Modelling assumptions of a particle:

Answer 2

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

Question 3

Rod:

Answer 3

All dimensions but one are negligible, like a pole or a beam

Question 4

Modelling assumptions of a rod:

Answer 4

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

Question 5

Lamina:

Answer 5

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

Question 6

Modelling assumptions of a lamina:

Answer 6

• Mass is distributed across a flat surface

Question 7

Uniform body:

Answer 7

Mass is distributed evenly

Question 8

Modelling 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

Light object:

Answer 9

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

Question 10

Modelling assumptions of a light object:

Answer 10

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

Question 11

Inextensible string:

Answer 11

A string that does not stretch under load

Question 12

Modelling assumptions of an inextensible string:

Answer 12

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

Question 13

Modelling assumptions of a smooth surface:

Answer 13

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

Question 14

Rough surface:

Answer 14

If a surface is not smooth, it’s rough

Question 15

Modelling assumptions of a rough surface:

Answer 15

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

Question 16

Wire:

Answer 16

Rigid thin length of metal

Question 17

Modelling assumptions of a wire:

Answer 17

• Treated as one-dimensional

Question 18

Smooth and light pulley:

Answer 18

All pulleys you consider will be smooth and light

Question 19

Modelling assumptions of a smooth and light pulley:

Answer 19

• Pulley has no mass
• Tension is the same on either side of the pulley

Question 20

Bead:

Answer 20

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

Question 21

Modelling assumptions for a bead:

Answer 21

• Moves freely along a wire or string
• Tension is the same on either side of the bead

Question 22

Peg:

Answer 22

A support from which a body can be suspended or rested

Question 23

Modelling assumptions of a peg:

Answer 23

• Dimensionless and fixed
• Can be rough or smooth (specified in question)

Question 24

Air resistance:

Answer 24

Resistance experienced as an object moves through the air

Question 25

Modelling assumptions for air resistance:

Answer 25

• Usually modelled as being negligible

Question 26

Gravity:

Answer 26

Force of attraction between all objects. Acceleration due to gravity is denoted by g. g = 9.8ms-2

Question 27

Modelling assumptions for gravity:

Answer 27

• 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 9.8ms-2, unless otherwise stated in the question

Question 28

SI Units

Answer 28

Question 29

gravitational force vs normal reaction

Answer 29

Question 30

Friction:

Answer 30

Friction is a force which opposes the motion between two rough surfaces

Question 31

Tension:

Answer 31

If an object is being pulled along by a string, the force acting on the object is called the tension in the string

Question 32

Thrust/Compression:

Answer 32

If an object is being pushed along using a light rod, the force acting on the object is called the thrust or compression in the rod

Question 33

Buoyancy:

Answer 33

Buoyancy is the upward force on a body that allows it to float or rise when submerged in a liquid

Question 34

Air resistance (in a force diagram):

Answer 34

Air resistance opposes motion

Question 35

What is a vector quantity?

Answer 35

A vector is a quantity which has both magnitude and direction

Question 36

What is a scalar quantity?

Answer 36

A scalar quantity has magnitude only

Question 37

Vector quantities:

Answer 37

Question 38

Scalar quantities:

Answer 38

Question 39

Magnitude (length)

Answer 39

• Distance is the magnitude (length) of the displacement vector
• Speed is the magnitude (length) of the velocity vector

Question 40

How can you describe vectors?

Answer 40

You can describe vectors using i, j notation, where i and j are positive 1 unit vectors in the x and y direction respectively