Mechanics

Question 1

Describe a particle in a model.

Answer 1

In a particle, all dimensions of the object are negligible.

Question 2

What modelling assumptions do we make about particles?

Answer 2

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

Question 3

Describe a rod in a model.

Answer 3

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

Question 4

What some modelling assumptions do we make about rods?

Answer 4

• Mass is concentrated along a line
• No thickness
• Rigid

Question 5

Describe a lamina in a model.

Answer 5

An object with area but negligible thickness, like a sheet of paper.

Question 6

What some modelling assumptions do we make about a lamina?

Answer 6

Mass is distributed across a flat surface.

Question 7

Describe a uniform body in a model.

Answer 7

Mass is distributed evenly

Question 8

What modelling assumptions do we make about uniform body’s?

Answer 8

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

Question 9

Describe a light object in a model.

Answer 9

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

Question 10

What modelling assumptions do we make about light objects?

Answer 10

• Treat object as having 0 mass
• Tension the same at both ends of a light string.

Question 11

Describe an inextensible string in a model.

Answer 11

A string that does not stretch under load.

Question 12

What modelling assumptions do we make about inextensible strings?

Answer 12

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

Question 13

What modelling assumptions do we make in smooth surfaces?

Answer 13

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

Question 14

Describe a rough surface in a model.

Answer 14

If a surface is not smooth, it’s rough.

Question 15

What modelling assumptions do we make about rough surfaces?

Answer 15

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

Question 16

Describe a wire in a model.

Answer 16

A rigid, thin length of metal.

Question 17

What modelling assumptions do we make about wires?

Answer 17

They are treated as 1D

Question 18

Describe a smooth and light pulley in a model.

Answer 18

All pulleys you consider will be smooth and light.

Question 19

What modelling assumptions do we make about smooth and light pulleys?

Answer 19

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

Question 20

Describe beads in a model.

Answer 20

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

Question 21

What modelling assumptions do we make about beads?

Answer 21

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

Question 22

Describe a peg in a model.

Answer 22

A support from which a body can be suspended or rested.

Question 23

What modelling assumptions do we make about pegs?

Answer 23

• Dimensionless and fixed
• can be rough or smooth as specified in question.

Question 24

Describe air resistance in a model.

Answer 24

Resistance experienced as an object moves through air

Question 25

What modelling assumptions do we make about air resistance?

Answer 25

Usually modelled as being negligible.

Question 26

Describe gravity in models.

Answer 26

Force of attraction between all objects.
Acceleration due to gravity is denoted by g.

Question 27

What modelling assumptions do we make about gravity?

Answer 27

• Assume all objects with mass are attracted towards earth.
• earths gravity is uniform and acts vertically downwards
• g is constant and is taken as 9.8m s^-2.