8. Modelling in mechanics

Question 1

What are the assumptions involved with particles?

Answer 1

Mass of the object is concentrated at a single point

Rotational forces and air resistance can be ignored

Question 2

What are the assumptions involved with a rod?

Answer 2

Mass is concentrated along a line
No thickness
Rigid

Question 3

What are the assumptions involved with lamina?

Answer 3

Mass is distributed across a flat surface

Question 4

What are the assumptions involved with a uniform body?

Answer 4

Mass of the object is concentrated at a single point at the geometrical centre of the body- the centre of mass

Question 5

What are the assumptions involved with a light object?

Answer 5

You treat the object as having zero mass

Tension is the same at both ends of a light string

Question 6

What are the assumptions involved with inextensible string?

Answer 6

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

Question 7

What are the assumptions involved with a smooth surface?

Answer 7

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

Question 8

What are the assumptions involved with a rough surface?

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 involved with a wire?

Answer 9

Treated as one-dimensional

Question 10

What are the assumptions involved with a pulley?

Answer 10

No mass
Tension is the same on either side of the pulley
No friction on the pulley

Question 11

What are the assumptions involved with a bead?

Answer 11

A particle with a hole in it for threading on a wire or string
Moves freely along a wire or string
Tension is the same on either side of the bead

Question 12

What are the assumptions involved with a peg?

Answer 12

A support from which a body can be suspended or rested
Dimensionless and fixed
Can be rough or smooth

Question 13

What are the assumptions involved with air resistance?

Answer 13

Usually deemed to be negligible

Question 14

What are the assumptions involved with gravity?

Answer 14

Assume all objects are drawn towards the Earth
Earth’s gravity is uniform and acts vertically downward
g is constant and is taken as 9.8 m/s2 unless otherwise stated

Question 15

What is weight?

Answer 15

The gravitational force of an object acting vertically downwards

Question 16

What is the normal reaction?

Answer 16

The force acting perpendicular to a surface when an object is in contact with the surface

Question 17

What is friction?

Answer 17

A force which opposes the motion between two rough surfaces

Question 18

What is tension?

Answer 18

The force acting on an object when it is being pulled along by a string

Question 19

What is thrust or compression?

Answer 19

If an object is being pushed along using a light rod, the force acting on the object is called the thrust or compression in the rod

Question 20

What is buoyancy?

Answer 20

The upward force on a body that allows it to float or rise when submerged in a liquid

Question 21

What is displacement?

Answer 21

The distance moved in a particular direction