Modelling Assumptions

Question 1

Particle - dimensions of the object are negligible

Answer 1

• mass of the object is concentrated at a single point

- rotational forces and air resistance can be ignored

Question 2

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

Answer 2

• mass is concentrated along a line
• no thickness
• rigid

Question 3

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

Answer 3

• mass is distributed across a flat surface

Question 4

Uniform body - mass is distributed evenly

Answer 4

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

Question 5

Light object- mass of the object is small compared to other masses, like a string or a pulley

Answer 5

• treat object as having zero mass

- tension the same at both ends of a light string

Question 6

Inextensible string - a string that does not stretch under load

Answer 6

-acceleration is the same in objects connected by a taut inextensible string

Question 7

Smooth surface

Answer 7

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

Question 8

Rough surface - if a surface is not smooth, it is rough

Answer 8

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

Question 9

Wire - rigid thin length of metal

Answer 9

• treated as one-dimensional

Question 10

Smooth and light pulley - all pulleys you consider will be smooth and light

Answer 10

• pulley has no mass

- tension is the same on either side of bead

Question 11

Bead - particle with a hole in it for threading on a wire or string

Answer 11

• moves freely along a wire or string

- tension is the same on either side of the bead

Question 12

Peg - a support from which a body can be suspended or rested

Answer 12

• dimensionless and fixed

- can be rough or smooth as specified in question

Question 13

Air resistance- resistance experienced as an object moves through the air

Answer 13

• usually modelled as being negligible

Question 14

Gravity - force of attraction between all objects. Acceleration due to gravity is denoted by g

Answer 14

• assume that all objects with mass are attracted towards the Earth
• Earth’s gravity is uniform and acts vertically downwards

Question 15

What is the value of acceleration due to gravity?

Answer 15

g = 9.81 m/s^2