3.4 Materials

Question 1

what is tensile force

Answer 1

forces that produce extension

Question 2

what is a compressive force

Answer 2

forces that shorten an object

Question 3

state Hooke’s law

Answer 3

the force of a load on a material is proportional to its extension provided the proportion limit is not exceeded

Question 4

what equation relates the force applied to a spring and its extension

Answer 4

F=kx

Question 5

springs in series and parallel

Answer 5

series: 1/Ktot= 1/K1 +1/K2 +…
parallel: Ktot= k1+k2+…

Question 6

what does the spring constant (force constant) tell you about the spring

Answer 6

its a measure of the stiffness of the spring

Question 7

explain what elastic limit means

Answer 7

if E is exceeded then material will not revert back to its original shape and size when the load is removed

Question 8

explain what plastic deformation means

Answer 8

permanent structural changes to the spring occur and it does not return to its original length when the force is removed

Question 9

how can you determine the elastic potential energy stored in a stretched elastic material from the force-extension graph

Answer 9

the area under the graph

Question 10

state two equations that can be used to calculate elastic potential energy

Answer 10

E=1/2Fx
E=1/2kx2

Question 11

How is Epe related to the extension

Answer 11

E is directly proportional to extension2

Question 12

If you double the extension of the spring, what happens to the energy stored

Answer 12

The energy stored quadruples

Question 13

What is a hysteresis loop

Answer 13

The loop formed by the loading and unloading curves (rubber band)

Question 14

Describe the molecular structure of a rubber band

Answer 14

Rubber consists of squashed and tangled long-chain molecules

Question 15

Define tensile stress

Answer 15

The force applied per unit cross sectional area of the wire

Question 16

State the equation for tensile stress

Answer 16

tensile stress= force/ cross sectional area (σ=F/A)

Question 17

What are the units of tensile stress

Answer 17

Pascals Pa

Question 18

Define tensile strain

Answer 18

Fractional change in the original length of the wire

Question 19

State the equation for tensile strain

Answer 19

Tensile strain= extension/original length (ε=x/L0)

Question 20

What are the units of tensile strain

Answer 20

Ratio or % so no unit

Question 21

What is meant by ‘ultimate tensile strength’

Answer 21

The maximum stress that a material can withstand when being stretched before it breaks

Question 22

Define Young’s modulus in words

Answer 22

Ratio of stress and strain

Question 23

State the equation for Young’s modulus

Answer 23

Young’s modulus= stress/ strain E=σ/ε

Question 24

What are the units for Young’s modulus

Answer 24

Nm-2 or Pa

Question 25

What is a polymeric material

Answer 25

a material that is composed of many monomers (from 10s to 1000s) all linked together to form chains.

Question 26

What is meant by the limit of proportionality

Answer 26

The region where the material obeys Hooke’s law

Question 27

What is a yield point on a stress-strain graph

Answer 27

Y1 and Y2 are lower and upper yield points- where the material extends rapidly

Question 28

What is the difference between ultimate tensile strength and breaking strength

Answer 28

UTS is the maximum stress that a material can withstand when being stretched before it breaks
Breaking strength is the stress value at the point of fracture of the material

Question 29

What is a ductile material

Answer 29

A material that can be easily drawn into a wire or hammered into a thin sheet

Question 30

What is a strong material

Answer 30

A material with a high ultimate tensile strength