SOLIDS UNDER STRESS

Question 1

Whose law states that extension of a spring is directly proportional to the force applied?

Answer 1

Hooke’s law

Question 2

When you double the force on a spring what happens to the extension?

Answer 2

It doubles

Question 3

What is the equation given by Hooke’s law?

Answer 3

Force = spring constant x extension (F = k x)

Question 4

What does k represent in Hooke’s law?

Answer 4

spring constant

Question 5

What does it mean if a spring has a high spring constant?

Answer 5

Very stiff

Question 6

What unit is used for spring constant?

Answer 6

Nm⁻¹ or Ncm⁻¹

Question 7

How do you find the extension of a spring?

Answer 7

Extension = Stretched length - Original length

Question 8

How can you find the work done in stretching a spring?

Answer 8

Area under force-extension graph (W=1/2 F x)

Question 9

What does the gradient (steepness) of a force-extension graph represent?

Answer 9

Spring constant

Question 10

What are the 3 types of solid?

Answer 10

Crystalline, Amorphous and Polymers

Question 11

What is the definition of a crystal?

Answer 11

Solid in which atoms are arranged in a regular array with long range order

Question 12

What is the definition of a crystalline solid?

Answer 12

Solid consisting of a crystals with short and long range order

Question 13

Give 2 examples of crystalline solids

Answer 13

Metals (polycrystalline) and Quartz

Question 14

What is the definition of an amorphous solid?

Answer 14

Atoms arranged randomly. No long range order

Question 15

Give 2 examples of amorphous solids

Answer 15

Brick and Glass

Question 16

What is the definition of a polymeric solid?

Answer 16

Solid made up of chain-like molecules

Question 17

Give 2 examples of polymers

Answer 17

Rubber and Nylon

Question 18

What is the definition of a ductile material?

Answer 18

A material that can be drawn out into a wire

Question 19

What is the difference between plastic and elastic materials?

Answer 19

Elastic materials return to original size and shape when a force is removed, Plastic materials are permanently deformed

Question 20

Sketch the stress-strain graph for a crystalline material

Answer 20

Straight line then curve. Elastic limit, elastic and plastic regions labelled.

Question 21

What is another name for a ductile fracture?

Answer 21

Necking

Question 22

What type of deformation do crystalline materials undergo?

Answer 22

Plastic deformation (necking)

Question 23

How can metals be strengthened?

Answer 23

Smaller grains or introducing foreign atoms

Question 24

Sketch the stress-strain graph for an amorphous material

Answer 24

Straight line. Elastic region only.

Question 25

What type of fracture does an amorphous material undergo?

Answer 25

Brittle fracture / Crack propagation

Question 26

How fast can cracks propagate in amorphous materials?

Answer 26

Speed of sound

Question 27

How can amorphous materials be strengthened?

Answer 27

Reduce imperfections (thin fibres) or put under compression (pre-stressed concrete)

Question 28

Sketch the stress-strain graph for a polymeric material

Answer 28

Steep straight line, long flat line, steep straight line

Question 29

Why is it hard to stretch a rubber band initially?

Answer 29

Van der Waals forces act over short distances to keep the molecules stuck together

Question 30

Why is it easy to stretch a rubber band after the initial section?

Answer 30

Molecule untangles and straightens out

Question 31

Why is it hard to stretch a rubber band in the final section?

Answer 31

Stretching/breaking strong covalent bonds requires a lot of force

Question 32

What is elastic hysteresis?

Answer 32

When the graphs of increasing and decreasing stress form a loop

Question 33

What is the area inside a hysteresis curve equal to?

Answer 33

Work done

Question 34

How can polymers be strengthened?

Answer 34

Cross-links (e.g. vulcanized rubber)

Question 35

What is stress?

Answer 35

Stress = Force / Area (σ = F/A)

Question 36

What are the units of stress?

Answer 36

Pa or Nm⁻²

Question 37

What is strain?

Answer 37

Strain = extension/original length (ε = Δl/l)

Question 38

What are the units of strain?

Answer 38

None

Question 39

What is Young modulus?

Answer 39

Young modulus = stress/strain (E = σ/ε)

Question 40

What are the units of Young modulus?

Answer 40

Pa or Nm⁻²