AP6 Materials and Performance

1. AP6 Materials and Performance Making the best use of available materials

2. Sorting materials into classes: • Metals (good conductors of heat and electricity, shiny, stiff, ductile, malleable) • Alloys • ceramics (insulators of heat and electricity, stiff, brittle); • polymers (insulators, often flexible, often plastic); • Wood / wood products • Composites • All of these may have slightly different mechanical properties at different temperatures • Qu. Match the pictures to the classes

3. Artefacts: • An object shaped or produced by human crafts from materials • materials are selected for their particular properties such as durability, cost, environmental impact, aesthetic appeal in these artefacts Qu1. list 6 artefacts found in your home and say how they can be described using the properties above. Qu2. Identify each artefact and its’ materials below. Suggest which properties they may have.

4. Working with materials can be risky… • Injuries and deaths can occur during the construction process, the testing and the occasional failure of the materials/ product

5. Looking out for our safety… We need to ensure a product/ artefact is… Safe, reliable and of a suitable quality The organisations below help to ensure we can trust the items we buy/ use • British Standards Institution (BS) • European Committee for Standardisation (CE) • International Organisation for Standards (ISO) These regulations and quality control are enforced by… • trading standards officer • building control surveyor • Health and safety inspectors

6. Dealing with the unexpected… • Most products have safety margins e.g. • Karabiners for climbing • Lifts • Strength of karabiner • Major axis 25kN• Gate open 9kN• Minor axis 7kN • These values are more than enough to dangle a few people let alone support a single climber (800N).

7. Choosing mechanical properties wisely… Qu 1. write a simple meaning for each term Qu 2. use the list to decide which properties the items below have… • Stiffness • flexibility • Toughness • brittleness • compressive strength • tensile (breaking) strength • hardness • density

8. L W (resourcefulphysics.org) (resourcefulphysics.org) Measuring the stiffness of materials: Making sense of the results… • Stiff materials will not change much as large forces are applied, or more graphically … have high gradients of the straight line part of the graph opposite… Lots of force needed for a small extension so stiff Very little force needed to cause a big extension so not stiff Force/N extension

9. Measuring the tensile (breaking) strength of materials: • The pulling force required to break a material [compressive strength uses pushing (squeezing) forces instead] Qu 1. Area is taken into account here. Which other variables might affect the results? Qu 2. Which point on the graph below gives the tensile strength? Force/N D C B A Extension/mm

10. Force-extension graphs • The area underneath the line gives us the energy stored (in this case 0.5x175Nx8m=700J) • The gradient gives us a constant k (in this case 100N/5m=20N/m) • F=kx Qu. Check to see if the constant above is correct by putting 2 more x values into the formula and seeing if they give the correct F values. F=force (N) and X=extension (m)

11. Plastic or elastic? • If objects return to their original shape after the force has been removed they are elastic • If objects stay deformed after the force has been removed they are showing plastic properties • Qu. What would happen if Reed Richards (Mr. Fantastic) stretches beyond his elastic limit?

12. Force-extension graphs 2 Material breaks Plastic section (curved) force Elastic section (straight) extension

13. Copper atoms Atoms of another element e.g. tin Alloys: • A solid solution e.g. Bronze, an alloy of copper and tin, is much harder than copper Alloying can be used to… Improve strength (e.g. high tensile steel) Make more resistant to corrosion (e.g. stainless steel) to lower or raise the melting point (e.g. solder)

14. Composite materials: • Have one material embedded in another one e.g. glass reinforced plastic • Glass fibres in a plastic matrix • Advantages include • low density • High strength • Easy to mould into shape • Used in boats, caravans, gliders • Uses the strength of glass with the flexibility/ ease of moulding of plastic making use of the good properties of each and minimising the bad properties of each

15. Best one for the job: • Matching mechanical properties... • E.g. lycra stretches as the wearer moves • E.g.2 climbing rope stretches as a large force is applied (the climber falls!) • Complementary mechanical behaviour (where 2 materials work in slightly different ways towards the same purpose)… • E.g. cycle helmet has a hard outer coating to spread the force and a spring inner to absorb the energy of impact • E.g.2 tiles are strong in compression and have a glazed coating to increase their hardness

16. Momentum • If there is a resultant force then there will be a change of momentum in the direction of the force • Momentum (kg m/s) = mass (kg) x velocity (m/s) • The change in momentum varies according to the resultant force and the time this force acts • Change in (kg m/s) = resultant force (N) x time force acts (s) momentum M m x v Mom mashes vegetables Draw your own revision triangle for this equation and make up your own mnemonic

17. Changing momentum • If there is no change in momentum the object stays at rest OR if moving stays at a constant speed in a straight line • During collisions a big change in momentum occurs but since… Change in (kg m/s) = resultant force (N) x time force momentum acts (s) If we increase the time over which the force acts then the resultant force will be smaller (the change in momentum is unchanged!) • This is the principle used in crash helmets, air bags, seat belts, climbing ropes and crumple zones on cars

18. Using Force-time graphs: • Area under graph equals the change in momentum • Qu. Calculate the change in momentum for the graph opposite • (hint: make two triangles and a box for the areas)

19. using Ft = m (v-u) • If F = mass x acceleration • And acceleration = change in velocity/t = (v-u)/t • F = change in momentum/t • Ft = final momentum-initial momentum • Ft = (mv) – (mu) • Ft = m(v-u) • Qu. A 1 kg remote control car stops after travelling at 10m/s in 5 seconds. What force was needed to do this? [hnt: rearrange the equation above to get F on its own]

20. Thermal Properties: • Thermal conductance… the ability of a material to allow heat energy to pass through • Aluminium saucepans let heat energy easily through but their wooden handles don’t • Thermal expansion… the increase in size of a material due to heating • Cold to the touch materials conduct heat away from our body • Warm to the touch materials are good insulators so hold the heat next to our skin • Qu. Give two examples for each of the last two bullet points

21. Thermal Behaviour: • Matchingto avoid gaps/ distortion when temperature changes occur e.g. camera body • Complementary e.g. bimetallic strip contains two different metals which expand at different rates when heated.

22. Electrical properties: • electrical conductance (G) is a measure of how easily electricity flows through a material • Can be measured simply using an ammeter and a voltmeter • Is the inverse of the resistance • G = I/V = 1/R • Must be high for the pins of a plug • Must be very low for the plastic casing of a plug

23. Acoustic properties: • More vibrations per second means a higher frequency (higher hertz) • Bigger displacements in the vibrations means louder (higher decibels) • Human hearing ranges from about 20 hertz to 20000 hertz (20 kHz) but is most sensitive at about 2000 hertz (2kHz)

24. The decibels scale: • A measure of loudness • note it is not a linear scale • A rise of 10 decibels means a doubling of loudness • the sound level of normal conversations is about 60 dB; • sound intensities above 85 dB can cause temporary hearing loss; • prolonged exposure to loud sounds can induce tinnitus and permanent hearing loss; • sound intensity above 130 dB causes pain

25. Noise: • Low frequency noise is less readily absorbed by surfaces so often causes more problems in buildings • Can be reduced by thick carpets, underlay, double glazing, ceiling tiles • Qu. Give two ways in which noise might affect someone’s health?

26. Reducing vibrations: Uses • rubber mats • Spring mountings e.g. shock absorbers • Fluid filled dampers

27. Optical Behaviour: • Qu. Match the properties to the diagrams… • Transparent • Reflective • Translucent • Opaque • Refraction

28. Refractive index: • A measure of how much light is slowed down inside a material which is also related to the amount of refraction (bending) and reflection (returned light) that occurs at its surface. Diamonds have a higher refractive index than common salt crystals so sparkle more

29. Specialised glass: • toughened glass (windscreens, fire doors) • infra-red reflectory glass (offices and eco homes) • self-cleaning glass (high rise offices) • lead glass (fancy glassware) • tinted glass (sunglasses) • photochromic glass (reactalight sunglasses) • High purity glass (optical fibres) Low purity here would cause light leakage from the fibres!

30. Lenses: • The focal length (f) will depend on the thickness of the lens, its’ shape and the refractive index Focal plane perpendicular to the optical axis Focal length

31. Images: • A representation of an object formed from rays of light which either cross (real image left below) or don’t cross (virtual image right below) • Images (I) can be… • made larger by moving the object closer to the lens • Inverted (see bottom left) or upright (see bottom right) • Moved past the focal length as objects are brought nearer

32. Eyesight: • The power of lenses are measured in Dioptres (inverse of focal length… so a powerful lens will have a short focal length) • Human eye = 60 Dioptres so parallel rays of light are brought to focus at 1/60th of a metre behind • Poor eyesight can be corrected by using glasses or contact lenses to account for lower/ higher power of the eye causing a blurred image on the retina

33. Contact lenses: • Problems… • Getting oxygen to the cornea • Seeing clearly • cleaned by the eye naturally • Stopping infections • Qu. Match the properties below to the problems above… stick to tears, transparent, easily sterilised, gas permeable • Qu 2. Name two more properties needed by contact lenses

34. Cameras: • Note… focal plane is perpendicular to the direction the camera is pointed • Lenses are often coated to work at lower light levels viewfinder lens Shutter opens and closes to let light through, hole formed is the aperture