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 tensile deformation?

Answer 4

Tensile deformation is when an object changes shape or properties under tensile forces

Question 5

What is compressive deformation?

Answer 5

Compressive deformation is when an object changes shape or properties after compressive forces are exerted

Question 6

What is Hooke’s Law?

Answer 6

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

Question 7

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

Answer 7

The limit of proportionality comes before the elastic limit

Question 8

How does an elastic body behave before the elastic limit?

Answer 8

Within the elastic limit, a spring experiences elastic deformation meaning it returns to original length when the force is removed

Question 9

How does an elastic body behave after the elastic limit?

Answer 9

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

Question 10

What is the relationship between force and extension?

Answer 10

Force is directly proportional to extension

Force = Spring Constant x Extension

Question 11

What is the spring constant?

Answer 11

The spring constant is a measure of the stiffness

Question 12

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

Answer 12

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 13

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

Answer 13

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

Question 14

What does it mean if springs are in parallel?

Answer 14

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

Question 15

What does it mean if springs are in series?

Answer 15

When two springs attached to each other hold up a mass

Question 16

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

Answer 16

Parallel springs:
k_total = k₁ + k₂

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

Question 17

How does energy behave within the elastic limit?

Answer 17

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

Question 18

How does energy behave during plastic deformation?

Answer 18

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

Question 19

What is the formula for work done on a spring?

Answer 19

Work done = Force x Extension

Area under force-extension graph

Question 20

What is the formula for elastic potential energy?

Answer 20

E = 1/2 * k * x²

E = 1/2 * F * x

Question 21

What is the relationship between elastic potential energy and extension?

Answer 21

E is directly proportional to x²

Question 22

What is tensile stress?

Answer 22

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

Question 23

What is the formula for tensile stress?

Answer 23

Tensile stress = Force / Cross-sectional Area

Question 24

What is the formula for tensile strain?

Answer 24

Tensile Strain = Extension / Original Length

Question 25

What is the ultimate tensile strength?

Answer 25

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

Question 26

What is the breaking strength?

Answer 26

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

Question 27

What is Young Modulus?

Answer 27

Young Modulus is the ratio of stress to strain

Question 28

What is relationship between stress and strain?

Answer 28

Stress is directly proportional to strain

Question 29

What is the formula for Young Modulus?

Answer 29

Tensile Stress / Tensile Strain Pa or Nm^-2

Question 30

What is the failure point?

Answer 30

Catastrophic failure total break of all bonds at one place

Question 31

What is the dislocation in a material?

Answer 31

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

Question 32

What is the force required at the yield point?

Answer 32

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

Question 33

How do the atoms in an elastic material act?

Answer 33

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 34

How do atoms in a plastic material act?

Answer 34

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 35

What are the effects of alloying?

Answer 35

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

Question 36

What are the properties of glass?

Answer 36

Brittle material Strong as little extension for force No plastic deformation

Question 37

What are the properties of steel?

Answer 37

Strong material Not ductile Breaks suddenly

Question 38

What are the properties of copper?

Answer 38

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

Question 39

What are the properties of plastic?

Answer 39

Plastic flow from the start Not strong Small elastic region

Question 40

What is plastic flow?

Answer 40

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

Question 41

What is strength?

Answer 41

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

Question 42

What is breaking stress?

Answer 42

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

Question 43

What is stiffness?

Answer 43

How difficult it is to change the shape of the object

Question 44

What is brittleness?

Answer 44

Stiff, but not strong

Question 45

What is elasticity?

Answer 45

Ability of a material to regain its original shape after it is distorted

Question 46

What is plasticity?

Answer 46

Characteristic of a material to not regain its shape after distortion

Question 47

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

Answer 47

More force is required to load the spring rather than unload

Question 48

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

Answer 48

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 49

How do you obtain an accurate diameter of a wire?

Answer 49

Taking multiple readings of the diameter along the wire

Question 50

How do brittle materials behave?

Answer 50

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

Question 51

What are polymeric materials?

Answer 51

Polymeric materials consist of long molecular chains

Question 52

What do polymeric materials behaviour depend on?

Answer 52

Molecular Structure Temperature

Question 53

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

Answer 53

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

Question 54

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

Answer 54

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

Question 55

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

Answer 55

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