Unit 4 Materials

Question 1

State Hooke’s law

Answer 1

Extension (or compression) of a spring is directly proportional to the force exerted on it

Question 2

List some properties of Hooke’s law.

Answer 2

Only valid when the limit of proportionality is not exceeded
Obeyed by wires and solid objects made of most materials
Can also be applied to the compression of a spring where the change in length is the amount the spring gets shorter.

Question 3

Define elastic deformation

Answer 3

A material returns to its original shape and size once any forces on it are removed

Question 4

Define plastic deformation

Answer 4

A material remains permanently deformed so does not return to its original shape and size once any forces on it are removed

Question 5

What is tensile stress?

Answer 5

The force stretching the object divided by its cross sectional area

Question 6

What is tensile strain?

Answer 6

The ratio of the object’s extension over its original length.

Question 7

What is plastic flow?

Answer 7

Where small stress leads to large strain because the cross sectional area of the material decreases rapidly

Question 8

What is UTS?

Answer 8

Ultimate tensile strain - maximum stress the material experiences, measure of the material’s strength

Question 9

What is a material’s breaking point?

Answer 9

The stress in the material at which it breaks

Question 10

List properties of stiff materials

Answer 10

Steep initial gradients
Large Young Modulus

Question 11

List properties of strong materials

Answer 11

High UTS and breaking point

Question 12

List properties of brittle materials

Answer 12

They break without much plastic deformation happening before breaking

Question 13

List properties of ductile materials

Answer 13

They undergo high plastic deformation before breaking

Question 14

What is the elastic limit of a material?

Answer 14

The point beyond which a spring or material becomes permanently deformed

Question 15

Draw + explain a force extension graph of a metal wire stretched beyond its elastic limit.

Answer 15

Unloading line does not go through the origin, wire is permanently stretched

Loading and unloading lines parallel because Young’s modulus is constant

Area between loading and unloading lines is the work done to permanently deform the wire

Question 16

Draw + explain a force extension graph of a rubber band

Answer 16

The unloading line goes through origin - rubber band returns to its original length

Area under loading curve is the work done to stretch rubber band

Area under unloading curve is the work done by rubber band when unloaded

Area between loading and unloading curves is the difference between energy stored in stretched rubber band and energy recovered when unstretched

Difference between energy stored and energy recovered is because some energy is transferred to the internal energy of the molecules

Question 17

Draw + explain a force extension graph of a polyethylene strip

Answer 17

Unloading line does not go through origin - polyethylene is permanently stretched

Area between loading and unloading curves is the sum of the work done to deform the strip and energy transferred to the internal energy of the molecules

Question 18

What does the area under the line of a force extension graph for an object represent?

Answer 18

Work done on object/ Total elastic potential energy stored by it

Question 19

What happens to the elastic potential energy of an elastically deformed object when the force on it is removed?

Answer 19

Transferred to other stores such as kinetic or gravitational potential

Question 20

What causes an object to be under tension?

Answer 20

Forces acting on it are stretching it

Question 21

What causes an object to be under compression?

Answer 21

Forces acting on it are compressing it

Question 22

What is the Young modulus of a material?

Answer 22

Measure of the material’s stiffness in Nm^-2 or Pa

Question 23

Define the limit of proportionality of a material

Answer 23

Point up to which its stress and strain are proportional

Question 24

How can the Young modulus of a material be found?

Answer 24

Gradient of the initial straight line section of its stress - strain graph

Question 25

What does the area under the straight line section of a material’s stress -strain graph represent?

Answer 25

Energy stored in the material per unit volume

Question 26

What does the first yield point on a material’s stress - strain graph represent?

Answer 26

Stress at which the material suddenly weakens and stretches plastically without any additional force

Question 27

How are crumple zones making cars safe?

Answer 27

They plastically deform during a crash so less energy is transferred to the passengers.