Mechanisms

Question 1

What are the modelling assumptions that can be made?

Answer 1

• Particle -> dimensions of the object are negligible
• Rod -> All dimensions but one are negligible, like a ole or a beam.
• Lamina -> Object with area but negligible thickness, like a sheet of paper
• Uniform body -> Mass is distributed evenly
• Light object -> Mass of the object is small compared to other masses, like a string or a pulley
• Inextensible string -> A string that does not stretch under load
• Smooth surface
• Rough surface -> If a surface is not smooth it is rough
• Wire -> Rigid thin length of metal
• Smooth and light pulley -> All pulleys you consider will be smooth and light
• Bead -> Particle with a hole in it for threading on a wire or string
• Peg -> A support from which a body can be suspended or rested
• Air resistance -> Resistance experiences as an object moves through the air
• Gravity -> Force of attraction between all objects. Acceleration due to gravity is denoted by g

Question 2

What assumptions can be made from 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 assumptions can be made from a rod?

Answer 3

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

Question 4

What is the assumptions made from Lamina ?

Answer 4

• Mass is distributed across a flat surface

Question 5

What is the assumption made from a light object?

Answer 5

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

Question 6

What are the modelling assumptions made from inextensible string?

Answer 6

• Acceleration is the same in objects connected by a taut inextensible string

Question 7

What assumptions are made from a smooth surface?

Answer 7

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

Question 8

What assumptions are made from the rough substance?

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

What are the assumptions made from a wire?

Answer 9

• Treated as one-dimensional

Question 10

What is the assumption made from a smooth and light pulley?

Answer 10

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

Question 11

What is the assumptions made from bead?

Answer 11

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

Question 12

What assumptions made from a peg?

Answer 12

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

Question 13

What are the assumptions made from air resistance?

Answer 13

• Usually modelled as being negligible

Question 14

What is the assumption made from gravity?

Answer 14

• 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 ms^-2, unless otherwise stated in the question