Modelling Assumptions

Question 1

Particle - Dimensions of the object are negligible - Modelling assumptions are?

Answer 1

• Mass concentrated at a single point.
• Rotational forces and air resistance ignored.

Question 2

Rod - All dimensions but one are negligible (i.e. pole or beam) - Modelling assumptions are?

Answer 2

• Mass concentrated along a line.
• No thickness.
• Rigid, does not bend or buckle.

Question 3

Lamina - Object with area but negligible thickness (i.e. sheet of paper) - Modelling assumptions are?

Answer 3

• Mass distributed across a flat surface.

Question 4

Uniform Body - Mass distributed evenly - Modelling assumptions are?

Answer 4

• Mass concentrated at a single point, at the centre of mass.

Question 5

Light Object - Mass is small relative to other masses (i.e. string or pulley) - Modelling assumptions are?

Answer 5

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

Question 6

Inextensible String - String that does not stretch under load - Modelling assumptions are?

Answer 6

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

Question 7

Smooth Surface - Modelling Assumptions are?

Answer 7

• Assume no friction between surface and any object on it.

Question 8

Rough Surface - If surface not smooth, it’s rough - Modelling assumptions are?

Answer 8

• Objects in contact with rough surface experience a frictional force, if moving or acted on by a force.

Question 9

Wire - Rigid thin length of metal - Modelling assumptions are?

Answer 9

• Treated as one-dimensional (width and thickness negligible).

Question 10

Smooth and Light Pulley - All pulleys you consider with be smooth and light - Modelling assumptions are?

Answer 10

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

Question 11

Bead - Particle with hole in it for threading on a wire or string - Modelling assumptions are?

Answer 11

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

Question 12

Peg - Support from which a body can be suspended or rested - Modelling assumptions are?

Answer 12

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

Question 13

Air Resistance - Resistance experienced as object moves through the air - Modelling assumptions are?

Answer 13

• Usually modelled as being negligible.

Question 14

Gravity - Force of attraction between all objects. Acceleration due to gravity denoted by g - Modelling assumptions are?

Answer 14

• All objects with mass are attracted to the Earth.
• Earth’s gravity is uniform and acts vertically downwards.
• g is a constant and taken as 9.8 (ms^-2), unless otherwise stated.