8.1/8.2 Modelling assumptions

Question 1

What does mechanics deal with?

Answer 1

» Motion and the action of forces acting on an object

Question 2

What are the assumptions for a particle?

»Model: Dimension of the object are negligible

Answer 2

» Mass of the object is concentrated at a single point
» Rotation forces and air resistance can be ignored

Question 3

What are the assumptions for a rod?

»Model: All dimension but one are negligible thickness

Answer 3

» No thickness
» Rigid - does not bend or buckle
» Mass is concentrated along a line

Question 4

What are the assumptions for Lamina?

»Model: Object with the area but negligible thickness

Answer 4

» Mass is distributed across a flat surface

Question 5

What are the assumption for a uniform body?

»Model: Mass is distributed evently

Answer 5

» Mass is concentrated at a single point called the centre of mass

Question 6

What are the assumptions of any light objects?

» Mass of the object is small compared to other masses, like a pulley

Answer 6

» Treat object has to have 0 mass
» Tension the same at both ends of a light string

Question 7

What are the assumptions for an inextensible string?

»Model: String does not stretch on load

Answer 7

» Acceleration is the same in an object connected by a taut inextensible string

Question 8

What are the assumptions for a smooth surface?

Answer 8

» There is no friction between the surface and any object on it

Question 9

What are the assumptions for a rough surface?

Answer 9

» Objects in contact with the surface experience a frictional force if they are acted on it by a force

Question 10

What are the assumptions for a wire?

»Model: It is a rigid thin length of metal

Answer 10

» Treated as one - dimensional

Question 11

What are the assumptions for a smooth and light pulley?

»Model: All pulleys will be considered smooth and light

Answer 11

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

Question 12

What are the assumptions for a bead?

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

Answer 12

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

Question 13

What are the assumptions for a peg?

»Model: A support from which a body can be suspended or rested

Answer 13

» Dimensionless and fixed
» Can be rough or smooth

Question 14

What are they assumptions for air resistance?

Answer 14

» Usually modelled as being negligible

Question 15

What are the assumptions for gravity?

»Model: Force of attraction between all objects is 9.8 m/s^2

Answer 15

» Assume all objects of 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