Section 5 - Materials

Question 1

What is density?

Answer 1

Mass per unit volume

Question 2

Does density vary with size or shape?

Answer 2

Neither

Question 3

What is the density of water?

Answer 3

1gcm cubed

Question 4

What does Hooke’s law say?

Answer 4

Extension is proportional to force

Question 5

What do tensile forces do?

Answer 5

Stretch the spring

Question 6

What do compressive forces do?

Answer 6

Squash the spring

Question 7

When does Hooke’s law stop working?

Answer 7

When the force exceeds the limit of proportionality

Question 8

What is the equation for Hooke’s law?

Answer 8

F = k delta L

Question 9

What is elastic deformation?

Answer 9

The material returns to its original shape and size

Question 10

What is plastic deformation?

Answer 10

The material is permanently stretched

Question 11

What happens to atoms in a material when it is put under tension in elastic deformation ?

Answer 11

The atoms are pulled apart from each other

Question 12

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

Answer 12

They return to their equilibrium distance part

Question 13

Is energy conserved while stretching?

Answer 13

Yes

Question 14

Is elastic deformation, what is the work done stored as?

Answer 14

Elastic strain energy

Question 15

What happens in elastic deformation after the stretching force is removed in terms of energy?

Answer 15

The elastic strain energy is transferred into other forms

Question 16

In plastic deformation, what is the work done stored as?

Answer 16

Energy is not stored as strain energy. Work is done to separate atoms

Question 17

In terms of energy is transferred, why are crumple zones good?

Answer 17

Some energy goes into changing the shape of the vehicle’s body as it deforms plastically, so less energy is transferred to the people inside

Question 18

What does a stress cause?

Answer 18

A strain

Question 19

What is the material called when a force can stretch it?

Answer 19

Tensile

Question 20

What is the material called when a force cannot stretch it?

Answer 20

Compressive

Question 21

Define tensile strain

Answer 21

The change in length divided by the original length of the material

Question 22

Define tensile stress

Answer 22

Force applied divided by the cross-sectional area

Question 23

What is the unit of strain?

Answer 23

It has no unit

Question 24

What is the unit of stress?

Answer 24

N/m^2

Question 25

What is breaking stress?

Answer 25

A stress big enough to break the material

Question 26

What is the UTS on a graph?

Answer 26

The ultimate tensile stress, the max stress that a material can withstand

Question 27

Describe the stress-strain graph for a material where the force is constantly increasing

Answer 27

Straight line through the origin, then starting to curve as it exceeds limit of proportionality, then UTS where the curve starts going down

Question 28

What does the area under a force-extension graph represent?

Answer 28

Elastic strain energy

Question 29

What is the Young's Modulus?

Answer 29

Stress/strain

Question 30

What is the unit of Young's Modulus?

Answer 30

N/m^2

Question 31

What does Young's Modulus measure?

Answer 31

The stiffness of a material

Question 32

Describe the experiment for Young's Modulus

Answer 32

- Get a long and thin wire - Measure cross-sectional area multiple times and average - Clamp wire to bench and add weights - Measure distance between fixed end of wire and marker - As weight increases the marker moves - Increase weight in steps - Calculate stressStrain using FL/A delta L

Question 33

What does the gradient of a stress-strain graph give?

Answer 33

Young's Modulus

Question 34

What are the 3 important points on a stress-strain graph?

Answer 34

- P is the limit of proportionality (when the graph is no longer a straight line) - Below P is the straight line through the origin which shows that the material is obeying Hooke's Law - E is the elastic limit, where the material starts to behave plastically - Y is the yield point where the material stretches without any extra load

Question 35

What type of line do you get for a brittle material on a stress-strain graph?

Answer 35

Straight line (no curves)

Question 36

Describe the stress-strain graph for a BRITTLE material where the force is constantly increasing

Answer 36

Straight line through the origin to show the material is obeying Hooke's law, then a sudden break in the material as it doesn't deform plastically

Question 37

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

Answer 37

F-E - specific for the tested object and depend on its dimensions S-S - Describe general behaviour of material, as stress and strain are independent of dimensions

Question 38

Describe the stages for a force-extension graph for a material where the force is constantly increasing

Answer 38

- Straight line through origin - Starts to curve when material exceeds elastic limit, and curves even more when the material starts to plastically deform - When load is removed, unloading line is parallel to loading line - Unloading line finishes at 0 force, but a longer extension as the material has been permanently stretched

Question 39

What does the area between a loading and unloading line on a force-extension graph represent?

Answer 39

Work done to permanently deform a wire