Modelling Assumptions

Question 1

what is a particle

Answer 1

dimensions of the object are negligible

Question 2

what do we assume when modelling 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 a beam

Question 4

what do we assume about 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 negligible
thickness is like a sheet of paper

Question 6

what do we assume about 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 do we assume about 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 do we assume about 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 do we assume about an inextensible string

Answer 12

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

Question 13

what is a smooth surface

Answer 13

exactly a smooth surface

Question 14

what do we assume about a smooth surface

Answer 14

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

Question 15

what is a rough surface

Answer 15

If a surface is not smooth, it is rough.

Question 16

what do we assume about a rough surface

Answer 16

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

Question 17

what is a wire

Answer 17

Rigid thin length of metal.

Question 18

what do we assume about a wire

Answer 18

treated as one-dimensional shape

Question 19

what is a smooth and light pulley

Answer 19

all pulleys you consider will be smooth and light

Question 20

what do we assume about a smooth and light pulley

Answer 20

pulley has no mass and tension is the same on either side of the pulley

Question 21

what is a bead

Answer 21

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

Question 22

what do we assume about a bead

Answer 22

moves freely along a wire or string
tension is the same on either side of the bead

Question 23

what is a peg

Answer 23

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

Question 24

what do we assume about a peg

Answer 24

dimensionless and fixed
can be rough or smooth as specified in question

Question 25

what is air resistance

Answer 25

Resistance experienced as an object moves through the air.

Question 26

what do we assume about air resistance

Answer 26

usually modelled as being negligible

Question 27

what is gravity

Answer 27

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

Question 28

what do we assume about gravity

Answer 28

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