Chapter 6 - Materials

Question 1

What are tensile forces

Answer 1

They produce extension

They undergo tensile deformation when tensile forces are exerted.

Question 2

What are compressive forces

Answer 2

They produce compression (shorten an object)

They undergo compressive deformation when compressive forces are exerted.

Question 3

Explain the force-extension graph for a spring

Answer 3

It is a straight line from the origin to the elastic limit. In this region, the spring undergoes elastic deformation. This means the spring can return to its original length.
Anything after the elastic limit, it will undergo plastic deformation. This is permanent and the spring can’t go back to its original shape when the force is removed.

Question 4

Hooke’s law

Answer 4

The extension of a spring is directly proportional to the force applied. This i true as the elastic limit of the spring is not exceeded.

Question 5

Equation for force constant

Answer 5

F= k x

k- force constant, x- extension

Question 6

What is the force constant

Answer 6

Essentially, it is a measure of the stiffness of the spring. A large force constant is difficult to extend.

Question 7

How can you investigate Hooke’s Law

Answer 7

Attach a spring to a clamp set up. Set up a metre ruler next to the spring. Keep adding masses from the spring and record the new length of the spring as you add each mass.

Question 8

How can you improve the accuracy of the length measurements when investigating the Hooke’s law

Answer 8

Using a set square, and by taking readings at eye level to reduce ‘parallax errors’

Question 9

What happens to the energy put into plastic deformation

Answer 9

If an object is deformed plastically, then the energy in work done can’t be fully recovered. Some has gone into moving the atoms’ arrangement into new, permanent positions. This energy isn’t recoverable

Question 10

work done equation

Answer 10

work done = force * extension

Question 11

What is the area underneath a force-extension graph

Answer 11

the work done

Question 12

What is the work done on a spring transferred to

Answer 12

It is transferred to the elastic potential energy within the spring. This is fully recoverable

Question 13

Elastic potential energy equation

Answer 13

E= 1/2 f x

E= 1/2 k x2

Question 14

tensile stress definition

Answer 14

the force applied per unit cross-sectional area

Question 15

tensile stress equation

Answer 15

force/ cross sectional area

Question 16

tensile stress unit

Answer 16

Pascals

Question 17

tensile strain definition

Answer 17

the fractional change in the original length of the wire

Question 18

tensile strain equation

Answer 18

extension/ original length

Question 19

Young Modulus definition

Answer 19

Within the limit of proportionality, stress is directly proportional to the strain. The ratio is constant, which is called the Young Modulus (E)

Question 20

Young Modulus equation

Answer 20

tensile stress/ tensile strain

OR
FL/ AX

Question 21

Young Modulus unit

Answer 21

Pascals

Question 22

How can you find E on a graph

Answer 22

On a stress- strain graph, it is the linear gradient of the graph

Question 23

What does E actually represent

Answer 23

The stiffness of the material

Question 24

On a stress-strain graph what is UTS

Answer 24

Ultimate Tensile Strength

the maximum stress a material can withstand being stretched before it breaks

Question 25

Brittle objects

Answer 25

Examples: iron or glass

They reach their elastic limit and will then break.

Question 26

Polymeric objects

Rubber bands

Answer 26

The loading curve is different from the unloading curve. The middle section is the energy released due to heating on a F-E graph. They don’t obey Hooke’s Law. They will eventually return to their original length.
This is mainly because more work is done when stretching a rubber band than is done when its extension decreases.

Question 27

What is the shape created by a rubber band called

Answer 27

A hysteresis loop

Question 28

Plastic Bags

Answer 28

They also don’t follow Hooke’s Law because they suffer plastic deformation.

Question 29

Metals

Answer 29

The loading curve is linear, and it follows Hooke’s Law, until a little bit past the breaking point.

Question 30

weighted average

Answer 30

(no. of thing * value of that thing) + (no. of another thing * value of that thing) / total number of things.

Question 31

what are yield points

Answer 31

points where the material extends rapidly . this type of curve may not happen for other ductile materials, but it does happen to steel wire.

Question 32

Read a graph of bending steel

Answer 32

Limit of proportionality 
Elastic limit 
Yield 1 
Yield 2
UTS 
Breaking point

Question 33

What does a higher UTS mean

Answer 33

that material is stronger

Question 34

what type of behaviour does rubber show

Answer 34

it shows elastic behaviour,

whereas polythene shows plastic behaviour

Question 35

what are polymeric materials

Answer 35

they consist of long molecular chains

Question 36

describe a graph of force against extension for a metal

Answer 36

If the forces are less than the elastic limit, then the unloading graph is equal to the loading graph. However, beyond the elastic limit, it may be permanently extended after the force is removed. it has suffered plastic deformation.

Question 37

describe a graph of force against extension for rubber

Answer 37

They don’t obey Hooke’s Law and it will return to its original position after the forces have been removed, but the loading and unloading graphs are different.

Question 38

describe a graph of force against extension for a polythene

Answer 38

It also doesn’t obey Hooke’s Law. The thin strips of polythene are quite easy to stretch so they can suffer plastic deformation under very little force.

Question 39

Example of when plastic deformation isn’t bad

Answer 39

Car frames are bent, and we wouldn’t want them to return into their original shape. For example, steel.