Modelling in mechanics

Question 1

3

What are the modelling assumptions of a particle? (dimensions of object are negligible)

Answer 1

• rotational forces and air resistance can be ignored
• mass of object is concentrated at centre
• the distance that the particle falls to the ground will not be exactly (h)

Question 2

3

What are the modelling assumptions of a rod?

Answer 2

• no thickness
• rigid (does not bend or buckle)
• mass is concentrated along a line

Question 3

1

What are the modelling assumptions of a lamina?

Answer 3

• mass is distributed across a flat surface

Question 4

1

What are the modelling assumptions of a uniform body?

Answer 4

• centre of mass - mass of object concentrated at single point at centre of body

Question 5

2

What are the modelling assumptions of a light object?

Answer 5

• object has zero mass
• tension is the same at both ends of a light string

Question 6

1

What are the modelling assumptions of an inextensible string (does not stretch under load)?

Answer 6

• acceleration is the same in objects connected by a taut inextensible strong

Question 7

1

What are the modelling assumptions of a smooth surface?

Answer 7

• no friction between surface and object

Question 8

1

What are the modelling assumptions of a rough surface?

Answer 8

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

Question 9

1

What are the modelling assumptions of a wire?

Answer 9

• treated as one-dimensional

Question 10

2

What are the modelling assumptions of a smooth and light pulley?

Answer 10

• pulley has no mass
• tension (in the string) is the same on either side of the pulley

Question 11

2

What are the modelling assumptions of a bead?

Answer 11

• moves freely along wire/string
• tension is the same on either side of the bead

Question 12

2

What are the modelling assumptions of a peg?

Answer 12

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

Question 13

1

What are the modelling assumptions of air resistance?

Answer 13

• modelled as negligible

Question 14

3

What are the modelling assumptions of gravity?

Answer 14

• assume all objects with mass are attracted towards earth
• Earth’s gravity is uniform and acts vertically downwards
• g is constant, g = 9.8

Question 15

3

What are common ways to improve a model?

Answer 15

• Use a more accurate value of g
• Account for air resistance
• Account for resistance force/friction

Question 16

What do you call the force that opposes weight on a floating object?

Answer 16

buoyancy