Model in Mechanics

Question 1

Particle

Answer 1

• Dimensions of the object are negligible
• Mass of the object is concentrated to a single point
• Rotational forces + air resistance can be ignored

Question 2

Rod

Answer 2

• All dimensions but one are negligible
• Mass is concentrated along a line
• Has no thickness
• It is rigid (does not bend or buckle)

Question 3

Lamina

Answer 3

• Object with area but negligible thickness (e.g. paper)
• Mass is disturbed across a flat surface

Question 4

Uniform body

Answer 4

• Mass is disturbed evenly
• Mass is concentrated at a single point at the centre of mass

Question 5

Light object

Answer 5

• Mass of the object is small compared to other objects
• Object has zero mass
  Its tension is the same on both ends of the string

Question 6

Inextensible string

Answer 6

• it does not stretch or load
• Acceleration is the same in objects connected by taut inextensible string

Question 7

Smooth Surface

Answer 7

• No friction on the surface of any object

Question 8

Rough Surface

Answer 8

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

Question 9

Wire

Answer 9

• Rigid thin length of metal
• it is one-dimensional

Question 10

Smooth and light pulley

Answer 10

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

Question 11

Bead

Answer 11

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

Question 12

Peg

Answer 12

• it is dimensionless and fixed
• can be rough or smoothed as specified in the question

Question 13

Air resistance

Answer 13

• Usually modelled as being negligible

Question 14

Gravity

Answer 14

• assume that all objects are attracted to earth
• Earth’s gravity acts vertically
• g is the constant for gravity and is taken as 9.8 unless otherwise stated in the question