5. Materials

Question 1

What is density?

Answer 1

‘compactness’ of a substance

Question 2

What is the approximate density of water at atmospheric pressure and room temperature?

Answer 2

1000 kgm^3

Question 3

What is the relationship between change in length and force applied?

Answer 3

change in length is directly proportional to force applied

Question 4

What is Hooke’s law?

Answer 4

force is directly proportional to the change in length up to the limit of proportionality

Question 5

What is the equation for Hooke’s law?

Answer 5

force = stiffness constant x change in length

Question 6

What is stiffness constant?

Answer 6

force needed to extend an object by 1 metre

Question 7

What does stiffness constant depend on?

Answer 7

• material
• length
• shape

Question 8

On a force against extension graph, what is the point where the straight line ends known as?

Answer 8

limit of proportionality

Question 9

How is the force shared when springs are arranged in parallel?

Answer 9

force is shared equally

Question 10

How is the force shared when springs are arranged in series?

Answer 10

they don’t share the force, they each carry the full weight

Question 11

What happens when an object undergoes an elastic stretch/deformation?

Answer 11

it returns to its original shape once the load has been removed

Question 12

What happens when an object undergoes an plastic stretch/deformation?

Answer 12

it doesn’t return to its original shape once the load is removed

Question 13

When does plastic deformation occur?

Answer 13

when you stretch an object past it’s elastic limit

Question 14

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

Answer 14

• work done to stretch the object by that change in length

- elastic potential energy now stored

Question 15

How are energy and extension related?

Answer 15

energy is directly proportional to extension squared

Question 16

What happens to the work put in to stretching a string in an elastic stretch when the force is removed?

Answer 16

it is released

Question 17

What happens so some of the energy in a plastic stretch?

Answer 17

• it is used to rearrange the bonds

- it is lost as heat

Question 18

What is stress?

Answer 18

force divided by cross-sectional area

Question 19

What is strain?

Answer 19

the change in length divided by the original length

Question 20

What is the gradient of the straight section of a force against extension graph?

Answer 20

stiffness constant

Question 21

What is the area under a force against extension graph?

Answer 21

elastic potential energy

Question 22

What is the gradient of the straight section of a stress against strain graph?

Answer 22

Young’s modulus

Question 23

What is the area under a stress against strain graph?

Answer 23

elastic potential energy per unit volume

Question 24

What is ultimate tensile stress?

Answer 24

the maximum stress a sample can withstand

Question 25

What is breaking stress?

Answer 25

the stress at which a material breaks

Question 26

What is Young’s modulus?

Answer 26

a measure of the stiffness of the material

Question 27

What is yield stress?

Answer 27

the force per unit cross-sectional area after which the material exhibits a large increase in strain for no increase in (or less) stress

Question 28

How does a brittle material break?

Answer 28

will break without warning and leave flat surfaces

Question 29

How does a ductile material break?

Answer 29

will deform and stretch slowly before breaking and leaving a deformed surface

Question 30

State two examples of brittle materials

Answer 30

glass, china

Question 31

State two examples of ductile materials

Answer 31

rubber, metals

Question 32

What is ‘necking’?

Answer 32

• where the object gets thinner
• concentrates force over a smaller cross-sectional area meaning force can be reduced but still cause an increase in strain

Question 33

What is brittle fracture?

Answer 33

when stress is applied, any tiny cracks on the material’s surface focus the stress and get bigger until the materials breaks completely leaving a flat break