Module 3.4 - Materials

Question 1

Hooke’s Law

Answer 1

Hooke’s law states that the extension of an object is proportional to the force that causes it, provided that the elastic limit is not exceeded

Question 2

Define elasticity

Answer 2

The property of a body to resume its original shape or size once the deforming force or stress has been removed

Question 3

Define deformation

Answer 3

The change in shape or size of an object

Question 4

Define plastic deformation

Answer 4

The material doesn’t return to its original shape/size after deformation

Question 5

Define elastic deformation

Answer 5

The object does return to its original shape/size after deformation

Question 6

How will an object get a positive extension?

Answer 6

Object has equal and opposite tensile forces on each end pulling away from one another creating tension, causing it to increase in length

Question 7

How will an object get compressed?

Answer 7

Object has two equal and opposite compressive forces acting towards each other causing compression, so the object will decrease in length/have a negative extension

Question 8

Define the limit of proportionality

Answer 8

The point at which the extension is no longer directly proportional to the force/load
The material will still behave elastically after this point, but not for much more load

Question 9

Define the elastic limit

Answer 9

Beyond this point the material will not show elastic behaviour
Any further load added will led to plastic deformation and the material will not return to its original shape

Question 10

Define plastic behaviour

Answer 10

Plastic deformation will be seen when the load is removed and the material wont return to its original shape

Question 11

Define fracture

Answer 11

The point at which the material will break

Before this happens, the material undergoes ‘creep’ where the planes of atoms slide past each other

Question 12

How would you investigate the stretching of a wire?

Answer 12

Set up a long, thin copper wire held firmly in a clamp at one end, the other end supporting a weight hanger after passing over a pulley (hanger must be just heavy enough to keep the wire taut)
Attach a marker to the wire and place a ruler in a fixed position below it
Add weights progressively to the hanger and record the mass, reading on ruler, wire extension and tension in a table (tension found using T = mg)

Question 13

Safety points about investigating stretching a wire

Answer 13

Wire may snap
Wear eye protection
Place box/cushion underneath hanger to catch the weight if it falls and to ensure you don’t stand directly underneath it

Question 14

Define force constant

Answer 14

The constant of proportionality in Hooke’s Law

Question 15

What is the area under the graph of a force-extension graph?

Answer 15

Work done

Question 16

What does the work required to stretch a material depend on?

Answer 16

Stretching force used

Distance moved in direction of force (extension)

Question 17

Define elastic potential energy

Answer 17

The energy stored in a stretched or compressed object

Question 18

Define stress

Answer 18

Force per unit cross-sectional area

Question 19

Define strain

Answer 19

Extension per unit length

Question 20

Two methods of calculating the Young modulus of a material

Answer 20

Gradient of a stress-strain graph

Directly from the equation

Question 21

Describe an experiment to calculate the Young modulus from a stress-strain graph

Answer 21

Set up Searle’s apparatus, measure the diameter of the wire using a micrometer and measure the original length of the wire using a ruler
Increase tension by adding weights to the right-hand side of the apparatus and measure the extension each time
Using this data, plot a graph of stress against strain and calculate the data to find the Young modulus of the wire
Use a vernier calliper to measure the extension as it’s so small

Question 22

Define ductile

Answer 22

Can be drawn into wires

Will show plastic deformation under tensile stress before breaking

Question 23

Define malleable

Answer 23

Can be hammered or beaten into flat sheets

Will show extensive plastic deformation when subjected to compressive forces

Question 24

Define brittle

Answer 24

Will break with little or no plastic deformation

Question 25

Define hard

Answer 25

Resist plastic deformation by surface indentation or scratching

Question 26

Define stiffness

Answer 26

The ability of a material to resist a tensile force

Question 27

Define polymeric

Answer 27

Made of long chains of molecules (polymers)

Question 28

Define ultimate tensile strength

Answer 28

Maximum stress it can withstand while being pulled or stretched, before it fails or breaks

Question 29

Why are polymeric materials (e.g. rubber) difficult to extend after a point, even if they don’t break?

Answer 29

The molecules are arranged in a mass of squashed long chains

When stress is applied, these chains straighten, resulting in a large strain

Question 30

How have the flexible properties of rubber been harnessed in the making of vehicle tyres?

Answer 30

Natural rubber becomes sticky and weak when warm, so is subjected to vulcanisation
This involves the addition of impurities (e.g. sulphur) to bind the chains of rubber molecules together making them harder and stronger - i.e. less easy to stretch