AS Mechanics: 8 Modelling in Mechanics

Question 1

What modelling assumptions can be made in a particle model?

Answer 1

• Mass of the object is concentrated at a single point.

- Rotational forces and air resistance can be ignored.

Question 2

What modelling assumptions can be made in a rod model?

Answer 2

• Mass is concentrated along a line.
• No thickness.
• Rigid (does not bend or buckle).

Question 3

What modelling assumptions can be made in a lamina model?

Answer 3

• Mass is distributed across a flat surface.

Question 4

What modelling assumptions can be made in a uniform body model?

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 modelling assumptions can be made in a light object model?

Answer 5

• Treat object as having no mass.

- Tension the same at both ends of a light string.

Question 6

What modelling assumptions can be made in an inextensible string model?

Answer 6

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

Question 7

What modelling assumptions can be made in a smooth surface model?

Answer 7

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

Question 8

What modelling assumptions can be made in a rough surface model?

Answer 8

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

Question 9

What modelling assumptions can be made in a wire model?

Answer 9

• Treated as one-dimensional.

Question 10

What modelling assumptions can be made in a smooth and light pulley model?

Answer 10

• Pulley has no mass.

- Tension is the same on either side of the pulley.

Question 11

What modelling assumptions can be made in a bead model?

Answer 11

• Moves freely along a wire or string.

- Tension is the same on either side of the bead.

Question 12

What modelling assumptions can be made in a peg model?

Answer 12

• Dimensionless and fixed.

- Can be rough or smooth as specifies in question.

Question 13

What modelling assumptions can be made in an air resistance model?

Answer 13

• Usually modelled as being negligible.

Question 14

What modelling assumptions can be made in a gravity model?

Answer 14

• Assume that all objects with mass are attracted towards the earth.
• Earth gravity is uniform and acts vertically downwards.
• g is constant and is takes as 9.8ms⁻² unless otherwise stated in the question.

Question 15

What is weight?

Answer 15

The force that acts vertically downwards.

Question 16

What is the normal reaction?

Answer 16

The force which acts perpendicular to a surface when an object is in contact with the surface.

Question 17

What is friction?

Answer 17

The force which opposes the motion between two rough surfaces.

Question 18

What is tension?

Answer 18

The force acting on an object being pulled along by a string.

Question 19

What is thrust/compression?

Answer 19

The force acting on an object being used along using a light rod.

Question 20

What is buoyancy?

Answer 20

The upwards force on a body that allows it to float/rise when submerged in a liquid.

Question 21

What is air resistance?

Answer 21

The resistive force that opposes motion.

Question 22

What is a vector quantity?

Answer 22

A quantity which has both magnitude and direction.

Question 23

Give examples of vector quantities.

Answer 23

Displacement (m): distance in a particular direction.

Velocity (ms⁻¹): rate of change of displacement.

Acceleration (ms⁻²): rate of change of velocity.

Question 24

What is a scalar quantity?

Answer 24

A quantity which has only magnitude.

Question 25

Give examples of scalar quantities.

Answer 25

Distance (m): the measure of length.

Speed (ms⁻¹): the measure of how quickly a body is moving.

Time (s): the measure of ongoing events taking place.

Question 26

Are scalar and vector quantities positive or negative.

Answer 26

Scalar: always positive.

Vector: either positive or negative.