7) Mechanical properties of solids and tissues

Question 1

Interactions between molecules of solids

Answer 1

Molecules of solids interact through Van Der Waals forces

The spacing of molecules is 10^-10

Density of solids is larger

Question 2

Spatial arrangements of molecules of solids

Answer 2

Determined by interaction forces between molecules

Most stable has minimum potential energy of interaction forces

Question 3

Crystal lattice

Answer 3

An arrangement of points repeating periodically in three dimensions

They consist of identical unit class that fill the space of the lattice

Question 4

Lattice order

Answer 4

Long range order
-arrangement of unit cells is continuous and unbroken

Short range order
-the crystal like structure is unbroken only for short distances

Question 5

Single crystal

Answer 5

The crystal lattice is continuous and unbroken in the whole body

Single crystals are anisotropic 
-properties are directionally dependent 
. Mechanical strength 
.thermal conductivity 
.velocity of light

Question 6

Polycrystals

Answer 6

Polycrystals- bodies made out of smaller monocrystals with random orientation

Polycrystals are isotropic- they’re homogenous in all directions

Question 7

Amorphous bodies

Answer 7

No repeating structures

Isotropic in all their properties

No definite boundary between solid and liquid states =supercooled liquids

Question 8

Classification of crystal lattices

Answer 8

230 space groups
32 classes
7 system

Polymorphism when a solid material can exist in more than one form of crystal lattice

Eg:carbon in graphite and diamond

Question 9

Classification of crystal lattice 2

Answer 9

Ionic
Metallic
Molecular
Network (atomic)

Question 10

Ionic crystal lattice

Answer 10

Salts have ionic crystal lattice

DRAWING
NACL

Question 11

Metallic lattice

Answer 11

A sea of delocalised electrons around positive charge

.high electrical conductivity
.high thermal conductivity

Eg: zinc

DRAWING

Question 12

Network crystal lattices

Answer 12

Elements in group 14 of the periodic table have network crystal lattices

eg:carbon

DRAWING

Question 13

Molecular crystal lattice

Answer 13

Crystal lattice is made by interaction of weak Van Der Waals forces

Eg: ice, co2 bromine and iodine

Question 14

Polymers

Answer 14

Long chains composed of repeating units called monomers

Protein is a naturally occurring polymer

Eg:cotton and skin

Question 15

Properties of polymers

Answer 15

Strong and capable of large elastic deformations

Very viscous

Don’t exist in gaseous state

Question 16

Conformations of polymers

Answer 16

Thermal motion and external forces cause reconfiguration of the polymer

Question 17

Liquid crystals

Answer 17

Liquid crystals

• can flow
• are anisotropic

Have narrow temp range

Properties are determined by structure or by shape

Question 18

Types of liquid crystals

Answer 18

Nematic
Smectic
Cholesteric

Question 19

Nematic

Answer 19

Long range orientational order

No positional order

Question 20

Smectic

Answer 20

Long range orientational order

Long range positional order In one direction

Question 21

Cholesteric

Answer 21

Arranged in layers

Orentiation is rotated by small angle from one layer to the next

Sensitive to external factors

Question 22

Deformation

Answer 22

Change in the size and shape of the body due to applied force

Mechanical properties of solids depend on their ability to resist deformation

Question 23

Types of deformation

Answer 23

Elastic
Then body recovers to its original shape and size

Plastic
Permanent deformation occurs

Question 24

Types of deformations 2

Answer 24

Classification:
Tensile 
Compression 
Bending 
Shear 
Twisting 

Examples
Muscles: stretching and compressing
Bones: stretching, bending and twisting

Question 25

Mechanism of elastic deformation

Answer 25

The distance between molecules change

Internal elastic forces arise trying to restore initial state

Question 26

Mechanical stress

Answer 26

Ó= F/S

F is deforming force
S is area of cross section

Question 27

Hooke’s Law

Answer 27

Applied stress ó and strain ę are directly proportional

Ó= Eę

E is Young’s modulus

Strain

ę=🔺L/L

Question 28

Modulus of elasticity

Answer 28

E is Young’s modulus and is a temperature dependant constant

Equal strain means higher stress in materials

Question 29

Deformation is crystalline solids

Answer 29

DRAWING

C is strength
B is elastic limit
A is proportional limit

Question 30

Other mechanical properties

Answer 30

Plasticity and brittleness

Hardness is determined by material resistance to scratching

Strength is determined by ultimate strength

Question 31

Deformation of polymers

Answer 31

Wrapping of long molecular chains is changed by deformation forces

Tensile force strengthens chain

Question 32

Elasticity of polymers

Answer 32

Rubber can experience strain of 300% without breaking but steel breaks at 0.3%strain

Question 33

Creep

Answer 33

Is the time dependant time stress and strain

Drawing x 3
F
ę
Ó

Question 34

Bone tissue

Answer 34

Made from hydroxyapatite and collage in ration 2:1

Fast deformation due to hydroxypatite and collages due to creep

Question 35

Skin

Answer 35

Main components
Collagen 75% and elastin 4%

Elastin stretches up to 300
Collage up to 10

Question 36

Muscles

Answer 36

Built of elastin and collagen

Deformation due to straightening of collagen molecules

Question 37

Walls of blood vessels

Answer 37

High collagen means less elasticity and higher strength

Elastin: collagen

2: 1 in carotid artery
1: 2 in the femoral artery