Chapter 6 - Materials

Question 1

What are tensile forces?

Answer 1

Tensile forces are equal and opposite forces acting on a material to stretch it

Question 2

What are compressive forces?

Answer 2

Compressive forces are two or more forces together that reduce the length of volume of an object

Question 3

What are restoring forces?

Answer 3

Restoring forces return a system to its equilibrium position

Question 4

What is Hooke’s Law?

Answer 4

Hooke’s Law states that the extension of a spring is directly proportional to the force applied

Question 5

In what order does the limit of proportionality and elastic limit come?

Answer 5

The limit of proportionality comes before the elastic limit

Question 6

How does an elastic body behave after the elastic limit?

Answer 6

Past the elastic limit, the spring undergoes plastic deformation and experiences permanent structural changes

Question 7

What is the relationship between force and extension?

Answer 7

Force is directly proportional to extension

Force = Spring Constant x Extension

Question 8

What is the spring constant?

Answer 8

The spring constant is a measure of the stiffness

Question 9

What are the steps of the Hooke’s Law Investigation?

Answer 9

Attach spring to one end using clamp, boss, and clam-stand secured to bench
Set up metre ruler
Suspend slotted masses form the spring
Record mass and new length of spring

Question 10

What are the precautions used for the Hooke’s Law Investigation?

Answer 10

Take readings at eye level reducing parallax errors
Use digital balance
Repeat each reading at least once
Obtain at least 6 recordings

Question 11

What does it mean if springs are in parallel?

Answer 11

When two springs hold up a mass from different points of connection

Question 12

What does it mean if springs are in series?

Answer 12

When two springs attached to each other hold up a mass

Question 13

How can the total spring constant be calculated for parallel and series?

Answer 13

Parallel springs:
k_total = k₁ + k₂

Series springs:
1/k_total = 1/k₁ + 1/k₂

Question 14

How does energy behave within the elastic limit?

Answer 14

When a material is compressed/extended within the elastic limit, work done on a material can be fully recovered

Question 15

How does energy behave during plastic deformation?

Answer 15

Plastic deformation moves atoms to new positions: energy is not recoverable

Question 16

What is the formula for work done on a spring?

Answer 16

Work done = Force x Extension

Area under force-extension graph

Question 17

What is the formula for elastic potential energy?

Answer 17

E = 1/2 * k * x²

E = 1/2 * F * x

Question 18

What is the relationship between elastic potential energy and extension?

Answer 18

E is directly proportional to x²

Question 19

What is tensile stress?

Answer 19

Tensile stress is the force applied per unit cross-sectional area of the wire

Tensile stress = Force / Cross-sectional Area

Question 20

What is the formula for tensile strain?

Answer 20

Tensile Strain = Extension / Original Length

Question 21

What is the ultimate tensile strength?

Answer 21

Ultimate tensile strength is the maximum stress that a material can withstand before it breaks

Question 22

What is the breaking strength?

Answer 22

The stress value at the point of fracture is the breaking strength

Question 23

What is Young Modulus?

Answer 23

Young Modulus is the ratio of stress to strain

Tensile Stress / Tensile Strain

Question 24

What is relationship between stress and strain?

Answer 24

Stress is directly proportional to strain

Question 25

What is the failure point?

Answer 25

Catastrophic failure total break of all bonds at one place

Question 26

What is the dislocation in a material?

Answer 26

The boundary between regions of a material that have slipped causing a misalignment of atomic planes

Question 27

What is the force required at the yield point?

Answer 27

At the yield point, the force required to move a dislocation is less that the force required to stretch atomic bonds

Question 28

How do the atoms in an elastic material act?

Answer 28

Metal cations increase in spacing as it is put under tension Atomic separation returns to initial value when force removed No energy lost in stretching

Question 29

How do atoms in a plastic material act?

Answer 29

Planes of atoms slide over each other Dislocations allow plastic behaviour at lower stress Dislocations allow bonds to be broken one at a time

Question 30

What are the effects of alloying?

Answer 30

Foreign atoms pin down dislocations making slips less likely Planes move over shorter distances

Question 31

What are the properties of glass?

Answer 31

Brittle material Strong as little extension for force

Question 32

What are the properties of steel?

Answer 32

Strong material Not ductile Breaks suddenly

Question 33

What are the properties of copper?

Answer 33

Strong material Ductile Deforms with plastic flow past elastic limit and yield points

Question 34

What are the properties of plastic?

Answer 34

Plastic flow from the start Not strong Small elastic region

Question 35

What is plastic flow?

Answer 35

Plastic flow occurs when a material under severe stress starts to behave as a fluid

Question 36

What is strength?

Answer 36

How much force is needed to break something Not a fair test as some things are thicker

Question 37

What is breaking stress?

Answer 37

Breaking Stress = Breaking Force / Area Force applied to the normal of an area

Question 38

What is stiffness?

Answer 38

How difficult it is to change the shape of the object

Question 39

What is brittleness?

Answer 39

Stiff, but not strong

Question 40

What is the relationship between the force as it is loaded and unloaded?

Answer 40

More force is required to load the spring rather than unload

Question 41

What does the area under a stress-strain graph show?

Answer 41

Total area is work done Area between x axis and unloading line is elastic potential energy Area between unloading and loading line is energy lost to heat

Question 42

How do you obtain an accurate diameter of a wire?

Answer 42

Taking multiple readings of the diameter along the wire

Question 43

How do brittle materials behave?

Answer 43

Brittle materials show elastic behaviour up to its breaking point without plastic deformation

Question 44

What are polymeric materials?

Answer 44

Polymeric materials consist of long molecular chains

Question 45

What do polymeric materials behaviour depend on?

Answer 45

Molecular Structure Temperature

Question 46

In what order do the key points appear on a stress-strain graph?

Answer 46

Limit of Proportionality Elastic Limit Yield Point 1 Yield Point 2 Ultimate Tensile Strength Breaking Point

Question 47

How do you tell from stress-strain graph that a material is brittle or ductile?

Answer 47

Brittle: Breaking point = UTS and little to no plastic deformation Ductile: Significant plastic deformation and exhibit yield points

Question 48

How do you tell from stress-strain graph that a material is elastic or plastic?

Answer 48

If it is elastic, it has a linear relationship. If it is plastic, it is non-linear

Question 49

What is the method for determining the young modulus?

Answer 49

Find average cross-sectional area of wire Attach metre ruler to workbench and place marker on wire at 0 Measure wire length Attach mass and record new position of marker Add additional mass and keep doing this for 7 readings Find stress and strain for each reading Plot and gradient is Young Modulus

Question 50

What is the method for investigating the property of plastic?

Answer 50

Slice plastic bag lengthways and widthways to test properties on each place Holepunch strips to hang masses from Measure strip length on clamp stand Add masses to strips and measure new length 10 readings