3.4.2.1 - Bulk Properties Of Solids

Question 1

Density =

Answer 1

Mass / volume

Question 2

What does the average density of an object determine?

Answer 2

Whether it floats or sinks - a solid object will float on a fluid if it has a lower density than the fluid.

Question 3

What is the density of water?

Answer 3

1 g cm-3

Question 4

What does Hooke’s law say?

Answer 4

The extension of a stretched object is proportional to the load or force.

Question 5

F =

Answer 5

kΔL - where k is constant

Question 6

What is the constant k?

Answer 6

The stiffness constant or in springs, the spring constant - it depends on the material being stretched.

Question 7

On a force extension graph where and what is P?

Answer 7

When the graph starts to curve, it is the limit of proportionality.

Question 8

What happens after the elastic limit is reached?

Answer 8

The material will be permanently stretched. When the force is removed, the object will not return to its original length.

Question 9

What does it mean if a deformation is elastic?

Answer 9

The material returns to its original shape and size once the forces are removed.

Question 10

What happens to the atoms when in an elastic deformation the material is put under tension?

Answer 10

The atoms of the material are pulled apart from one another.

Question 11

Why in elastic deformation do the atoms return to their original position?

Answer 11

Atoms can move small distances relative to their equilibrium positions, without actually changing position in the material.

Question 12

What happens to the atoms in elastic deformation when the load is removed?

Answer 12

The atoms return to their equilibrium distance apart.

Question 13

Under what conditions does elastic deformation occur?

Answer 13

As long as the elastic limit of the object isn’t reached.

Question 14

Stress =

Answer 14

Force / area

Question 15

Strain =

Answer 15

Change in length / original length

Question 16

Cross sectional area =

Answer 16

πd^2 / 4

Question 17

What are the unit for strain?

Answer 17

It has no units, it is a ratio.

Question 18

What happens to the stress/strain equations if the forces producing the stress and strain are tensile or compressive?

Answer 18

There is no difference to the equations, but you tend to think of tensile forces as positive and compressive forces as negative.

Question 19

What happens to atoms as stress increases?

Answer 19

The effect of stress is that the the atoms start to pull apart from one another.

Question 20

What happens if the stress is too great?

Answer 20

The atoms separate completely, and the material breaks, this is called the breaking stress.

Question 21

What is the ultimate tensile stress?

Answer 21

The maximum stress that the material can withstand.

Question 22

Why do engineers have to consider the ultimate tensile stress and breaking stress of materials?

Answer 22

They need to make sure the stress on a material won’t reach the ultimate tensile stress when the conditions change as breaking stress and UTS are dependant on conditions such as temperature.

Question 23

Where can you find the elastic strain energy?

Answer 23

Under a force-extension graph

Question 24

What happens to the work done when stretching a material?

Answer 24

All the work done is stored as elastic strain energy in the material.

Question 25

What is the energy stored in a stretched wire equation?

Answer 25

1/2 kΔL^2

Question 26

What does plastic deformation mean?

Answer 26

Material is permanently stretched, therefore doesn’t return to its original shape.

Question 27

What does a force-extension graph look like for brittle objects?

Answer 27

It obeys Hooke’s law for a certain period of time but then it fractures so the line stops, there is no curving as they don’t deform plastically.

Question 28

What has to be done in order to stretch a material?

Answer 28

Work done

Question 29

What happens to the work done if the deformation is elastic?

Answer 29

All the work done is stored as elastic strain energy in the material.

Question 30

What happens to the stored elastic strain energy when the stretching force is removed?

Answer 30

The energy is transferred to other forms.

Question 31

What happens to energy during plastic deformation?

Answer 31

The work done is to separate atoms and energy is not stored as strain energy.

Question 32

How is plastic deformation used in transport design?

Answer 32

Crumple zones are designed to deform plastically in a crash. Some energy goes into changing the shape of the vehicles metal body, and so less is transferred to the people inside.

Question 33

On a stress strain graph what happens at the yield point?

Answer 33

The material suddenly starts to stretch without any entrance load. The yield point is the stress at which a large amount of plastic deformation takes place with a constant or reduced load.

Question 34

What happens at the elastic limit?

Answer 34

The material starts to behave plastically. From this point on the material would no longer return to its original shape once the stress was removed.

Question 35

What happens at the point of proportionality?

Answer 35

The graph is no longer a straight line, as the material stops obeying Hooke’s law. However it would still return to its original shape if the stress was removed.

Question 36

On a stress-strain graph what does a straight line indicate?

Answer 36

The material is obeying Hooke’s law. The gradient of the line is constant - it is the Young modules.

Question 37

What does the area underneath the straight line on the stress-strain graph indicate?

Answer 37

The energy stored in the material per unit volume.

Question 38

What do brittle materials look like on a stress-strain graph?

Answer 38

A straight line through the origin and they don’t curve as they fracture.

Question 39

What is the difference between force-extension graphs and stress-strain graphs?

Answer 39

Force-extension graphs are specific for the tested object and depend on its dimensions. Stress-strain graphs describe the general behaviour of a material because stress and strain are independent of dimensions.

Question 40

What can you plot unloading on?

Answer 40

A force-extension graph

Question 41

When unloading, why would the line not go back to the origin?

Answer 41

Because the material as been stretched beyond the its elastic limit and so has deformed plastically.

Question 42

What does the area between the loading and the unloading lines represent?

Answer 42

The work done to permanently deform the wire.