5: Materials

Question 1

What is the equation for density?

Answer 1

mass/volume

Question 2

What is Hooke’s law?

Answer 2

the extension of a stretched wire is proportional to the load or force

Question 3

what is the equation for Hooke’s law?

Answer 3

F = kΔL

Question 4

What do tensile forces do to a string?

Answer 4

Stretch the spring

Question 5

What do compressive forces do to a spring?

Answer 5

Squash the spring

Question 6

What is represented by the gradient of a force-extension graph?

Answer 6

Spring constant

Question 7

What is meant by the elastic limit?

Answer 7

The point where the force applied to an object becomes so great it no longer obeys Hooke’s law, and the graph starts to curve

Question 8

What happens if you increase the force past the elastic limit?

Answer 8

• the material will be permanently stretched

- When all the force is removed, the material will be longer than at the start

Question 9

What is meant by the limit of proportionality?

Answer 9

• The point at which a material no longer obeys Hooke’s law

Question 10

How could you investigate how extension varies with force?

Answer 10

• The object should be supported at the top, e.g. using a clamp, and a measurement of its original length should be taken using a ruler
• Weights should be added one at a time to the other end of the object
• The weights used will depend on the object being tested - a trial investigation should be completed to work out the range and size of the weights needed
• After each weight is added, the extension should be calculated
• extension = new length - original length
• A graph should then be plotted

Question 11

What does it mean if a deformation is elastic?

Answer 11

The material returns to its original shape once the forces are removed - so it has no permanent extension

Question 12

How does elastic deformation work in terms of atoms?

Answer 12

• when a material is put under tension, the atoms of a material are pulled apart from one another
• atoms can move small distances relative to their equilibrium positions without actually changing position in the material
• Once the load is removed, the atoms return to their equilibrium distance apart

Question 13

What does it mean if a deformation is plastic?

Answer 13

• the material is permanently stretched after the force is removed
• Some of the atoms in the material move position relative to each other
• When the load is removed, the atoms don’t return to their original positions

Question 14

What is meant by tensile stress?

Answer 14

the force applied, F, divided by the cross-sectional area

- stress = F/A

Question 15

What is meant by tensile strain?

Answer 15

• the change in length (extension) divided by the original length
• ΔL/L

Question 16

How does stress work in terms of atoms?

Answer 16

• It starts to pull atoms apart from one another

- The stress eventually becomes so great that atoms separate completely, and the material breaks

Question 17

What is meant by the breaking stress?

Answer 17

The stress great enough to break a material

Question 18

what is UTS?

Answer 18

• ultimate tensile stress

- the maximum stress a material can withstand

Question 19

What is represented by the area under a force-extension graph?

Answer 19

Elastic strain energy

Question 20

the energy stored by the stretched material is equal to…

Answer 20

The work done on the material in stretching it

Question 21

What is the equation for elastic strain energy?

Answer 21

E = 1/2FΔL

Question 22

Energy changes in an oscillating spring can be summed up as:

Answer 22

change in kinetic energy = change in potential energy

Question 23

What is the purpose of crumple zones in cars?

Answer 23

• They deform plastically in a crash
• This means some of the car’s kinetic energy goes into changing the shape of the vehicle’s body, so less is transferred to the people inside

Question 24

What is the Young’s modulus?

Answer 24

The constant produced by stress/strain when a material has not reached its limit of proportionality

Question 25

What is the equation of the Young modulus?

Answer 25

E = FL/AΔL

Question 26

How could you investigate the Young’s modulus of a material?

Answer 26

• Use a long, thin wire to reduce uncertainty
• Find the CSA of the wire (micrometer)
• Clamp the wire to the bench so you can hang weights off one end of it
• Start with the smallest weight necessary to straighten up the wire
• Measure the distance between the fixed end of the wire and the marker
• if you increase the weight, the marker moves
• Increase the weight in steps, recording the marker reading each time - the extension is the difference between this and the original length
• Use results to work out stress and strain then plot a graph
• The Young modulus is the gradient of the graph

Question 27

What is meant by the yield point of a material?

Answer 27

• the point at which a material starts to stretch without any extra load
• the stress at which a large amount of plastic deformation takes place with a constant or reduced load

Question 28

Why is the loading line parallel to the unloading line on a F-ΔL graph?

Answer 28

the spring constant remains the same

Question 29

How do you know a wire has been stretched past its elastic limit and deformed plastically on a F-ΔL graph?

Answer 29

• The unloading line doesn’t go through the origin

Question 30

What would the stress-strain graph look like for a brittle material?

Answer 30

• A straight line that does not curve
• Always goes through the origin, always obeys Hooke’s law
• Same for F-ΔL graphs

Question 31

Do brittle materials deform plastically?

Answer 31

no

- When stress reaches a certain point, the material snaps

Question 32

What is meant by brittle fracture?

Answer 32

• when a stress is applied to a brittle material cracks at the material’s surface get bigger and bigger until the material breaks completely
• These cracks are able to grow due to the rigid structure of the material