School Science Lessons
2024-06-24d
Materials
(UNPh34)
Contents
34.1 Bridges
34.2 Cement
Preparations
Tests for strength
34.3 Coade stone
34.4 Coefficient of restitution. bounce
34.5 Earthquakes, Richter scale
35.5.0 Elements in the Earth's crust
34.6 Environment pollution
35.6.0 Geological time scales
34.7 Heat treatment of metals
34.8 Mechanical properties of materials
34.12 Oxyacetylene welding
34.8.6 Wire
34.1 Bridges
34.6.1 Classification of bridges (gif)
34.6.4 Truss systems (gif)
34.6.3 Simple cantilever, used in building porches, (gif)
Experiment
34.1.1 Test the strength of a simple bridge
34.2 Cement
34.2.1 Mortar, sand and slaked lime, sand and cement
34.2.2 Portland cement
34.2.3 Tests for cement brick strength (contents)
34.2.4 Tests for cement brick strength (water content)
34.2.5 Tests for cement change in weight when setting
34.66.3 Tests for concrete alkalinity
Preparations
34.17 Prepare quicklime, CaO
34.18 Prepare slaked lime. Ca(OH)2
34.19 Prepare putty
Tests for strength
34.65.0 Tests for strength of mud, clay, and sand bricks
34.67.0 Tests for strength of plaster of Paris bricks
34.3 Coade stone
Coade stone, "lithodipyra", was a popular artificial stone used before the invention of Portland cement, used for statues and sculptures.
It was famed to resist weathering and many statues and facades remain unweathered today.
It was made from a secret recipe of silica, alumina, flint, quartz, clay and soda-lime glass.
34.4 Coefficient of restitution, bounce
Experiment
34.4.1 Bouncing balls, Silly putty, silicone, bouncing putty, "Tricky Putty"
34.4.2 Coefficient of restitution, (coefficient of elasticity)
34.4.3 Dead and live balls
34.5 Earthquakes, Richter scale
Earthquakes are violent moving or shaking of the Earth's caused by geological forces or volcanic activity.
Seismic waves are the vibrations from earthquakes that travel through the Earth.
They are recorded on seismographs that record a zig-zag trace of the amplitudes of ground oscillations.
Seismographs can detect earthquakes from sources anywhere in the world.
The Richter Magnitude Scale (Charles F. Richter, USA, 1935), an open-ended scale, is used to compare the size of earthquakes from the logarithm of the amplitude of waves recorded by seismographs, adjusting for the distance between seismographs and the epicentre of the earthquakes.
Each point on the scale presents a ten-fold increase in magnitude.
Magnitude is expressed in whole numbers and decimal fractions, e.g. strong earthquake magnitude 6.3.
The more recent Moment Magnitude Scale is a logarithmic scale of 1 to 10 that enables seismologists to compare the energy released by different earthquakes on the basis of the area of the geological fault that ruptured in the earthquake.
34.6 Environmental pollution
Experiment
34.6.1 Electrostatic precipitation
34.6.2 Noise sources, Test A
34.6.3 Noise control, Test B
34.6.4 Pollution from light of buildings
34.6.5 Pollution from noise, noise effects, thinking and learning, white noise
34.7 Heat treatment of metals
Heating steel material to "red heat" then cooling it slowly is called annealing.
Putting steel material heated to red heat into cold liquid to cool it quickly is called quenching.
Reheated steel material is quenched to the temperature slightly lower than "red heat" temperature, then cooled slowly, called tempering.
Annealing, quenching and tempering are heat treatings to change rigidity, brittleness and toughness, by changing the arrangement of iron atoms.
Annealing is a form of heat treatment to soften a metal and make it easier to work.
Annealing is often used to soften steel to relax its inner stress to change its shape by forging, pressing and machining.
Experiment
34.7.1 Annealing
34.7.2 Heat treatment of needles
34.7.3 Heat treatment of razor blades or steel knitting needles
34.7.4. Heat treatment of steel needles, annealing, quenching, tempering
34.7.5 Quenching
34.7.6 Tempering
34.8 Mechanical properties of materials
34.8.1 Mechanical properties of materials, elastic, ductile, malleable
34.8.2 Breaking strains, brittleness
34.8.3 Bulk modulus, modulus of incompressibility, K
34.8.4 Elasticity (Stress: tension, compression, shear)
34.8.5 Hooke's law, elastic limit, deforming force, stress and strain
34.8.6 Poisson's ratio, v
34.8.7 Shear modulus, modulus of rigidity, G
34.8.8 Stress and strain
34.8.9 Viscoelasticity, creep
34.8.6 Wire
34.8.11 Young's modulus, E, elasticity, stress and strain
Experiments
34.8.12 Test solid models, sphere packing
34.8.13 Test strength of paper, shape and its mechanical strength
34.8.14 Use coffee, coffee tins
34.8.15 Use crystals
34.8.16 Use ice
34.8.17 Use lip balm
34.8.18 Use nail polish
34.8.19 Use nitinol memory wire
34.8.20 Use springs
34.15.14 Use toothpaste
34.15.15 Use toothpicks
34.12 Oxyacetylene welding
Oxyacetylene welding, (oxy-acetylene welding)
In excess air, acetylene (ethyne), burns with a hot white flame.
If the acetylene is mixed with oxygen from separate gas cylinders in an oxy-acetylene torch, a temperature of 3 500oC can be produced to allow welding of tubes and pipes.
The two ends of the metal objects to be welded are heated until they begin to melt.
Rods of "filler" metal are melted along the joint then the joint cools and solidifies.
The oxyacetylene flame can burn under water.
Also, metal can be preheated with the oxyacetylene flame then cut with a pure oxygen flame.
2(C2H2 (g) + 5O2 (g) ---> 4CO2 (g) + 2H2O (g)
Products
Arc welding electrodes, Satin craft 13, 5 kg, 3.2 mm, 380 mm, pack
Arc welding electrodes, Satin craft 13, 5 kg, 2.5 mm, 300 mm, pack
Arc welding electrodes, GP 6012, 5 kg, 3.2 mm, 380 mm, pack
Arc welding electrodes, GP 6012, 5 kg, 2.5 mm, 300 mm, pack
Arc welding mig wires, automatic, autocraft LWI, 15 kg spool, 0.9 mm
Welding gloves, chrome leather, black and gold
Welding goggles, for oxy-acetylene welding, lift up front
Welding helmet, lift up front, head harness
Welding mats, anti-fatigue, non-slip mats, resistant to welding sparks, 700 mm x 800 mm centre piece
Welding mats, anti-fatigue, non-slip mats, resistant to welding sparks, 700 mm x 800 mm end piece
Welding safety apron, chrome leather, reinforced straps, 910 mm x 560 mm
Wire / scratch brush, four rows of steel bristles with wooden handle
34.8.8 Stress and strain
1. Stress, tectonic stress is the deforming force per unit area, and is measured in pascal (Pa) and pound per square inch (psi).
2. Tensile stress causes elongation, stretching, and occurs when rocks are pulled apart, causing them to lengthen or break apart at divergent plate boundaries.
3. Confining stress occurs when rocks are pushed down by the rocks, so they cannot move and cannot deform.
4. Compressive stress causes compression, squashing, and occurs when rocks are squeezed together, to fold or fracture at convergent plate boundaries.
5. Bulk stress, (or volume stress), causes squeezing on all sides, and occurs deep in the ocean.
6. Shear stress is caused by tangental push and pull forces, and occurs when forces are moving in parallel opposite directions, at transform plate boundaries.
7. Strain describes the deformation caused by stress as a fraction, so it has no units.
A strain gauge is a sensor whose resistance varies with applied force converted into a change in electrical resistance which can then be measured.
Stress is the object's internal resisting forces, and strain is the displacement and deformation that occur.
Experiment
Strain gauge: Pull to various lengths a spring attached to a dynamic force transducer and show the resulting force on a voltmeter.
Responses to increasing stress:
8. Elastic deformation occurs when the rock returns to its original shape when the stress is removed.
9. Plastic deformation occurs when rocks under stress do not return to its original shape when the stress is removed.
10. Fracture occurs when a rock under stress breaks.
Strain increases with increase in stress.
11. Elastic modulus = stress / strain.
The same stress on elastic band, (smaller elastic modulus), causes greater deformation than on a same size steel band, (greater elastic modulus).
12. Young’s modulus is the elastic modulus for tensile stress, e.g. Copper 117, Iron 210, Steel 200, Tin 47.
Young's modulus
13. Hooke's law: The deformation is directly proportional to the deforming force or load, within the elestic limit.
Hooke's Law
Experiment
34.13.3 Plywood sheets, shear torsion
34.13.1 Shear book, foam block
34.13.4 Torsion rod, modulus of rigidity, bending and twisting
34.15 Tensile and compressive stress
Hooke's law, (Commercial).
Experiment
34.15.1 Bend beams, bend metre stick, stress, rectangular bar, woods
34.15.2 Breaking spaghetti
34.15.3 Breaking threads
34.15.4 Prince Rupert's drops, tempered glass, toughened glass
34.15.5 Sagging board, aluminium / steel elasticity paradox
34.15.6 Shear strength of thin sheets
34.8.10 Wire
Local purchase
Connecting wire, insulated copper wire, connecting cord, single conductor cable
Copper wire
Fly wire, roll
Fuse wire, 5A, reel 100 g
Fuse wire, 15A, reel 100 g
Iron wire, fence wire
Iron wire, tie wire
Wire, bell wire or lamp wire, 100 m
Wire cable, 3-core, 6 A, 1 metre
Wire, copper wire, bare, 16 swg, 500 g reel
Wire, copper wire tinned, PVC cover, reel 100 m
Wire, Eureka wire / constantan, 16 swg, 125g
Wire, flexible wire, single wire, piano wire, 10 m
Wire helix, spring, slinky spring
Wire netting, chicken wire
Wire, nichrome wire, bare, 16 swg, reel 125g
Wire, platinum wire, for flame test
Wire stripper
34.17 Prepare quicklime, CaO
The word "lime" is commonly used for both quicklime and slaked lime.
Quicklime is manufactured by roasting chalk or limestone in a lime kiln.
It emits a brilliant light when strongly heated, and was used for lighting stages, hence the phrase "to be in the limelight".
Quicklime, burnt lime, CaO, is prepared from marble, chalk, or limestone, CaCO3
1. Use a lump of marble, chalk (not blackboard chalk), or limestone twice the size of a thimble, and 20 cm of iron wire.
Copper wire is not suitable, because it melts with the heat.
The wire used for tying up bundles of firewood answers the purpose.
Tie one end of the wire round the lump and hold the other end in a pair of pliers or fasten it in a metal stand.
Put a sheet of asbestos or a metal tray below the Bunsen burner in case the lump falls out of the wire.
Suspend the lump just inside a very hot flame and heat it for ten to fifteen minutes.
In a short time the lump begins to glow as quicklime forms.
After heating let the lump to cool on the asbestos or metal tray.
2. Put the lump of marble, chalk or limestone into a glowing fire with a pair of tongs and leave it there for twenty minutes.
Quicklime can also be made from powdered chalk with the help of a home-made blowpipe.
CaCO3 --> CaO + CO2
Calcium carbonate --> calcium oxide + carbon dioxide.
This lime-burning occurs above about 830oC.
After cooling, the quicklime is slowly converted back to calcium carbonate because of the revers reaction with the carbon dioxide in the air.
34.18 Prepare slaked lime. Ca(OH)2
Slaked lime, or calcium hydroxide, is made from quicklime by adding water to the latter.
This process is called "slaking" the quicklime.
Slaked lime is used to make limewater and mortar.
It is also used by gardeners to "sweeten" the soil, i.e. increase the pH.
Slaked lime, E526, is a saturated solution of calcium hydroxide, and is also called limewater or milk of lime or killed lime.
. Put small lumps of fresh quicklime into an evaporating dish or watch glass.
Use a test-tube to add drops of water.
Clouds of steam arise, accompanied by a hissing noise.
The water may boil.
Finally, the solid breaks up into a fine, dry powder.
The water has combined chemically with the quicklime, and slaked lime remains.
The word "quick" in quicklime means "live", as in "the quick and the dead" and "quicksands".
The superstitious people of the middle ages believed that quicklime was inhabited by a "spirit".
When water was added to quicklime the "spirit" was released and a "dead" substance remained.
CaO + H2O --> Ca(OH)2
Calcium oxide + water --> calcium hydroxide
The solution can cause burns.
The white calcium hydroxide powder is not very soluble in water and as it dissolves it emits heat, leaving the rest in suspension.
Its solubity decreases wiht temperature, called retrograde or reverse solubility.
34.19 Prepare putty
Prepare putty (glazier's putty, painter's putty), with calcium carbonate paste (whiting), + linseed oil (+ white lead).
Putty is used as a, filler in glazing, to seal glass into frames.
34.2.3 Tests for cement brick strength (contents)
See diagram 3.65: Test the strength of a brick.
See diagram 3.2.66: Cardboard box for cement test.
Cement is any material that binds loose sediment into a rock and may be ferruginous (containing iron), calcareous (containing calcium), and siliceous (containing silica).
Builders' cement contains calcium and aluminium silicates.
Concrete contains aggregate (gravel and sand), cement, and water.
Concrete can be cast into shape to become load bearing.
Mortar contains sand, cement and water and is used for plasters.
Grouts contain cement and water and are used to fill gaps.
Experiment
Make 5 boxes out of stiff paper or cardboard 1.5 cm deep, 5 cm wide and 10 cm long.
Use adhesive tape or clips to fasten the edges.
A cement brick, the same size as the clay bricks, can be cast in these boxes.
Smear a little oil or grease around the inside surfaces of the boxes.
Obtain some fresh Portland cement from a builder.
1. Cement / water brick
Mix the cement with water to a thick paste and fill the box with it, smoothing off the top surface level with the paper.
It should "set" in a few minutes, but it will take a few days to "harden".
"Setting" is to change from a fluid to a firm rigid material, but a mark can still be scratched on the surface with a nail.
To "harden" is to become rock hard.
2. Cement / sand / water brick
Mix 1 part of cement powder with 3 parts of clean sand.
Work into a thick paste with water.
Pour into the paper box, smooth off the surface and leave to set and harden.
3. Cement / sand / gravel / water brick
Make a brick as before using 1 part cement powder, 1 part of sand, 3 parts clean gravel and water.
Cast the brick and leave to set and harden.
This is a concrete brick.
4. Cement / lime / sand / water brick
A builder buys quicklime and mixes this with water to make calcium hydroxide on the building site just before he uses it.
Mix 1 part of cement, 5 parts of builders' lime, calcium hydroxide, and 2 parts of sand and make into a paste with water.
Cast a brick as before and leave to harden.
5. Test the strength of the above bricks.
34.2.4 Tests for cement brick strength (water content)
See diagram 3.65: Test the strength of a brick.
Wrap the waste cement in newspaper then put it in waste containers.
Do not pour cement paste down the sink.
Use identical cardboard milk cartons for moulds.
Mould 1. Put 200 mL of a mixture of dry cement and sand in a large beaker.
Slowly add 100 mL of water from a measuring cylinder, with stirring, to the mixture until it becomes a thick paste.
Pour the paste into cardboard mould.
Smooth the surface of the cement in the mould.
Wipe out the beaker with paper and rinse with water.
Record the volume of water used.
Mould 2. Repeat the experiment with 20% less water than in Mould 1.
Pour the paste into cardboard mould.
Mould 3. Repeat the experiment with another 20% less in water.
Pour the paste into cardboard mould.
Mould 4. Repeat again with 20% more water than in Mould 1.
Pour the paste into cardboard mould.
Mould 5. Repeat 1. and pour the paste into cardboard mould 5.
Cover the cardboard Moulds 1 to 4 with plastic wrap to prevent evaporation, but leave Mould 5 uncovered.
Leave all the moulds in a warm place for 2 days.
Examine the above mixtures:
* Note the surfaces.
* Scratch the surfaces with your fingernail, a nail, and the point of a file.
* Hardness tests
Hardness test 1. Drop a steel ball from the same height on the surfaces, while wearing safety glasses, and note the bounce height.
The harder the surface, the greater the bounce height.
Hardness test 2. Remove the cardboard and use a hammer to hit each mixture with increasing intensity until it breaks.
Wear safety glasses when you do this.
2. Record the order of surface hardness by both methods and the resistance to breaking.
Note the relative hardness and the volume of water used.
Note the relative hardness of mould 1 and mould 5.
3. Repeat the experiment with the ratio of sand to cement from 50 mL of sand + 150 mL of cement, to 50 mL of cement + 150 mL of sand.
Test the strength of the above bricks.
34.2.6 Tests for concrete alkalinity
Concrete is an artificial stone used as a building material.
It contains cement, sand, water and an aggregate, crushed stone or slag, a mixture of oxides formed during ore smelting and refining.
Reinforced concrete uses steel bars, twist bars, or cables to counteract weakness in tension.
Alkaline cement protects steel reinforcing rods in concrete from corrosion.
Clean pieces of steel reinforcing or nails with sandpaper and put them into two jars half filled with water.
Put broken pieces of concrete in one jar.
After a week, note that the steel in the jar without the concrete corrodes faster.
Carbon dioxide from the atmosphere slowly penetrates the surface of concrete and reacts with lime, Ca(OH)2, to convert it to limestone, CaCO3, reducing the alkalinity of the concrete touching the steel bars.
The steel can form rust containing iron oxides and hydroxides that have a larger volume than iron.
This expansion cracks the concrete.
Find a broken piece of old exposed concrete.
Break it and wet the new surface with phenolphthalein indicator solution.
A pink coloration indicates the high alkalinity inside the concrete with a rim of untinted concrete around the edge.
34.2.5 Tests for cement change in weight when setting
Weigh 500 g of sand and cement mixture (industrial mortar mix), into a polystyrene drink cup and add 75 grams of water.
Mix the contents until all the lumps are gone.
Weigh the polystyrene cup and contents again to check the weight of the added amount of water.
Fill another polystyrene cup with water to the same level and weigh the cup + water.
Leave the polystyrene cups for one day then weigh them again.
The loss in weight of the cup + water only shows the loss by evaporation of the cup + cement mixture + water.
The rough surface area of the setting concrete does allow water to evaporate faster than in cup + water only.
However, the loss by evaporation is negligible.
The experiment shows that most of the added water is absorbed in the chemical reaction of the setting cement.
34.2.1 Mortar, sand and slaked lime, sand and cement
Use 5 mL of slaked lime and 20 mL of clean sand.
Wash sea sand four times with water to get rid of the salty impurities.
Put the slaked lime into an old cup and make it into a paste with water.
Stir in the sand at a time, adding more water as needed, until a stiff paste forms.
Scrape out the paste on to a tin lid and leave it for a day or two.
It will set into a hard mass.
For a basic mortar, mix three parts of sand for every one part of cement.
34.2.2 Portland cement
1. Portland cement hardens as it reacts with water.
It was thought to have the same colour as stone on Isle of Portland, U.K.
Portland cement is a fine powder produced by grinding Portland cement clinker and some gypsum.
2. The raw mixture is mainly chalk or limestone containing clay or silicon dioxide and other materials, including clay, shale, sand, iron ore, bauxite. fly ash and slag,
i.e. minerals containing calcium oxide, silicon oxide, calcium aluminate, aluminium oxide, ferric oxide, and magnesium oxide.
Calcium and silicon form the calcium silicates that give strength to the concrete.
Aluminium and iron compounds produce the liquid solvent flux in the kiln that helps in the formation of silicates at a conveniently low temperature.
3. The raw mixture is heated in a cement kiln at 1400-1450 oC, the ingredients become sintered, about 1/3 melted, but not fused into a molten mass.
It cools to become grey-coloured clinker containing at least two thirds by weight of calcium silicates.
Calcium sulfate as gypsum is added to the clinker.
The gypsum hydrates very rapidly during the concrete setting reaction and helps to control the initial setting rate.
The mixture is ground to form fine cement powder that can be stored dry and later mixed with water to form an alkaline cement workable slurry for casting.
4. Portland cement powder may contain 50% tricalcium silicate, 3(CaO).SiO2, 25% dicalcium silicate, 2(CaO).SiO2,.
10% tricalcium aluminate, 3(CaO).Al2O3, 10% tetracalcium aluminoferrite, 4(CaO)4.Al2O3.Fe2O3, 5% gypsum, CaSO4.2H2O.
Portland cement: about 65% calcium oxide, CaO, 25% silicon oxide, SiO2, 5% aluminium oxide, Al2O3, 1% ferric oxide, Fe2O3, 4% calcium sulfate, CaSO4.
Different types of cement contain the same four major compounds that make up at least 90% of the total weight, but in different proportions.
Tricalcium silicate + water (yields) --> calcium silicate hydrate + calcium hydroxide + heat.
5. When water is added to concrete powder, hydration occurs and during this chemical reaction the concrete gradually hardens as calcium silicate.
hydrate gel that forms in the first few days at the surface and later deeper in the pour.
The strength of hard concrete comes from the solid part of the paste, the calcium silicate hydrate and other crystalline phases.
The pores remaining in hard concrete are filled with water and air and have no strength.
dicalcium silicate + dicalcium silicate + water --> calcium silicate hydrate + calcium hydroxide + heat
The volume of setting concrete should not change, because the added water should be used up in the hydration process.
So the weight of cement powder + water + aggregate = weight of the set concrete block (conservation of mass).
The water-cement ratio (by weight) of completely hydrated cement is 0.22 to 0.25, excluding evaporable water.
So the warning "Do not touch wet concrete until it dries" is inaccurate because nearly all the water is lost of the hydration reaction, not by evaporation.
The rate of reaction of the cement with water is proportional to the surface area of the particles.
Cement production requires high energy input and produces large quantities of carbon dioxide, so it contributes to global warming.
EMC, Energetically Modified Cement, uses very finely ground ingredients, so increased surface area for the chemical reaction and uses less energy to produce it.
34.7.4 Heat treatment of steel needles, annealing, quenching, tempering
1. Annealing is used to produce a soft state in worked metals.
Heat a needle to bright red heat.
Hold it vertically in the flame and then take one minute to raise it slowly out of the flame.
Leave to cool.
Try to bend the needle with a pair of pliers.
The needle should now be soft.
You can easily bend it around a pencil.
2. Quenching is used to make steel metals harder and non-ferrous metals softer, e.g. copper.
Heat a needle to bright red heat and immediately plunge it into cold water.
Try to bend the needle with a pair of pliers.
The needle should now be brittle.
You can easily break it into small pieces.
3. Tempering of steel is reheating after rapid cooling to give extra secondary harness.
Heat a needle to bright red heat and immediately plunge it into cold water.
Use 5 cm sewing needles that are tough and springy and difficult to bend.
They are made of an alloy of iron with a small proportion of carbon.
Clean and shine the surface of the needle with emery cloth.
Heat the needle very gently until a deep blue oxide film appears on the surface.
This colour indicates the tempering temperature of the needle.
Leave to cool.
Try to bend the needle with a pair of pliers.
The needle is tough and springy again.
34.65.0 Tests for strength of mud, clay, and sand bricks
See diagram 3.65: Test the strength of a brick.
1. Find a source of clay soil or mud.
If it is dry, it must be mixed with water.
To do this, put about 350 mL of water in a suitable container such as a plastic bowl.
Crush the dry clay to a powder and then mix it with water until a thick smooth paste forms.
Squeeze it through your fingers until no lumps remain.
It will have the correct consistency when it is thick and pliable and sticks more to itself than to your fingers.
Spread the clay or mud on to a flat surface very evenly to make a slab of 1.5 cm thickness.
Use a clean wet knife to cut four bricks, each 10 cm by 5 cm.
Dry one under the sun for two or three days and bake another by a fire.
Try making a sand brick of the same size.
2. Use a brick (house brick), sold by a building contractor.
See diagram 3.65: Test the strength of a brick.
Examine the bricks for cracks.
Test whether the surface comes away by rubbing with a dry finger.
Test whether the surface comes away by rubbing with a wet finger.
Test the strength of the small 5 X 10 X 1.5 cm bricks.
Support the two ends of the brick on the edges of two tables.
Load the middle of the test brick with weights or attach a bucket into which sand can be poured.
Keep loading until the test brick breaks.
Suspend the weights and bucket near the floor so that they have almost no distance to fall.
34.67.0 Tests for strength of plaster of Paris bricks
See diagram 3.65: Test the strength of a brick.
Plaster of Paris is partially dehydrated calcium sulfate crystals, CaSO4.H2O, made by heating gypsum.
When mixed into a paste with water it sets quickly and expands.
It is used as a fine casting material.
Put 4 mL of water into a beaker.
Add the powdered plaster of Paris slowly with a spatula.
Continue adding the plaster until it just appears above the surface of the water.
The plaster absorbs the water and you should finish with a very thin layer of water, about 1 mm, above the plaster.
Stir the mixture well.
When it begins to thicken, pour it into the paper box.
Smooth the surface of the cement in the mould and leave to set for 1 day.
Test the surface and strength of these bricks.
Plaster of Paris is not often used as a construction material, but calcium sulfate as gypsum, CaSO4.2H2O, is used to prepare Portland cement.
34.7.1 Annealing
1. Heat a needle to bright red heat.
Hold it vertically in the flame and then very slowly raise it out of the flame taking about one minute.
When it is cool, try bending it.
It should be soft and easily bent round a pencil.
2. Use pliers to clamp a needle's tail and forcibly insert a needle into the hard block then try to bend the needle.
You may find it is very difficult, because the needle has strong rigidity and toughness.
Now use the pliers to clamp its tail and place it on an alcohol burner to heat.
About one minute later, its most part changes dark red.
Lay it aside to cool slowly.
When its temperature lowers to the room temperature, insert it into the block.
You may find that it is easy to bend it.
34.7.2 Heat treatment of needles
Obtain some sewing needles about four to 5 cm long.
These needles are alloys of iron and carbon, but the proportion of carbon is very small.
Try bending a needle.
It is tough and springy.
These properties of this carbon steel are dependent on the arrangement of the carbon atoms among the iron atoms.
The effect of annealing, quenching and tempering is to alter this arrangement in a specific way.
34.7.3 Heat treatment of razor blades or steel knitting needles
The properties of steel depend on the manner in which the steel has been treated previously and how it has been heated or cooled.
Experiment
1. Hold one end of a razor blade in a pair of pliers and try to bend the other with a pair of pincers.
The blade snaps, because it is brittle, although the steel is extremely hard.
2. Hold one corner of a razor blade in a pair of pliers and heat it strongly over a Bunsen burner flame until it is red hot.
When it has been red hot for half a minute make the flame gradually less hot and smaller, so that the blade cools down very slowly.
The gradual cooling should occupy at least five minutes.
When the blade is cold it is found to have lost its hard and brittle character.
It can now be bent easily without breaking, and it stays bent.
This process of slow cooling is called "annealing" the steel.
3. Straighten the blade used in the foregoing experiment, and once more heat it until it is red hot.
Have available cold water in an old cup or mug.
When the blade has been red hot for a short time put it into the cold water.
The rapid cooling in this treatment makes the blade hard and brittle.
4. Dry the blade after the quick cooling in the previous experiment.
Rub it with emery paper until the surface is bright and clean.
Holding the corner of the blade in the pliers.
Heat it by holding it an inch above a medium Bunsen burner flame until a blue sheen just appears over the surface.
Let the blade cool.
It is now strong and springy.
This moderate heating followed by cooling is called "tempering" the steel.
34.7.5 Quenching
Quenching. 1. Neither the soft needle nor the brittle needle is very useful.
However, the tough springy form can be restored.
Heat and quench a needle as before to obtain the hard, brittle form.
Carefully clean and shine the surface with emery cloth.
The needle must now be heated very gently until a deep blue oxide film appears on the surface.
This colour is an indication of the temperature at which the needle is tempered.
When the needle is cool, try bending it.
It is inot tough and springy like the original needle.
2. Heat a needle to bright red heat and, while it is still hot, plunge it completely into cold water.
Try to bend it now.
It should be brittle and easily broken into small pieces.
3. Use the pliers to clamp the tail of another needle and heat it on an alcohol to dark red.
Place it into cold water at a beaker to cool it quickly.
Insert it into the block then bend it.
You may find that it becomes very hard, but brittle and easy to break.
34.7.6 Tempering
Tempering: Polish a quenched needle, as above 34.7.5, with emery cloth,r then reheat it on a spirit burner.
When it becomes blue black, take it from the burner and lay it aside to cool slowly.
When its temperature lowers to the room temperature, insert it into the block to bead it.
You may find that it becomes tough.
34.8.1 Mechanical properties of materials, elastic, ductile, malleable
34.8.2 Breaking strains, brittleness
34.8.3 Bulk modulus, modulus of incompressibility, K
34.8.4 Elasticity (Stress: tension, compression, shear)
34.8.9 Viscoelasticity, creep
34.8.1 Mechanical properties of materials, elastic, ductile, malleable
See diagram 3.2.62: Metal punch
See diagram 34.5.0: Three tubes, originally of the same length.
In the diagram, the three bars were originally all the same length.
They demonstrate the concepts of stress, strain, Poisson's ratio and the strength of a material.
Stress: force per unit area
Strain: deformation produced by stress
Poisson's ratio: the ratio of the proportional decrease in a lateral measurement to the proportional increase in length of a stretched substance.
1. Elastic
If forces are applied to a body remaining in equilibrium, the length volume or shape alters temporarily or permanently, i.e. it becomes deformed.
If the forces applied to the body stop and the body regains its original length, volume and shape, then deformation occurred within the elastic limit of the body.
The magnitude of the elasticity of the body or the material comprising the body is expressed as a modulus of elasticity.
2. Ductile
Ductility is the ability of metals or alloys to keep their strength and be permanently distorted and not crack or fracture, when their shape is altered.
Some ductile metals, e.g. copper, can be drawn through a die to reduce the cross-section by plastic flow and form wire.
However but other metals lose their strength and crack.
Gold is among the most ductile metals.
One gram of gold can be drawn into a wire 2 km long.
The atoms of a ductile metal can slide past each other without causing the material to break into pieces.
A ductile metal can be hammered so finely that light can pass through it.
Only metals are ductile.
3. Malleable
A malleable metal can be hammered, pressed or extruded out of the original shape, and not tend to return to the original shape or to fracture or break.
Both ductile and malleable metals or alloys have large crystals.
Metals have a regular pattern of fixed particles consisting of the nucleus of an atom and inner electrons around the atom.
Outer electrons (delocalized electrons, valence electrons) are held loosely by the nuclei of the atoms. Metals are malleable and ductile, because distorting metallic crystals does not completely break all the metallic bonds.
Many metals have high melting points and high boiling points, because their chemical bonds are strong.
The greater the number of outer shell valence electrons the higher the boiling point.
34.81.4 Elasticity (Stress: tension, compression, shear)
Tensile test machine, (Commercial).
Structures Tester, stress and strain, (Commercial).
"Balloon Racer", use elastic potential energy to power a racing car (toy product).

Tenacity (Geology)
Most minerals are brittle and may be classified as follows:
1. Sectile (cuts easily with a knife)
2. Malleable (can be flattened out under a hammer)
3. Flexible (can be bent without breaking)
4. Elastic (natural shape remains after expansion, contraction or distortion)
Elastic materials strain when stretched and quickly return to their original state once the stress is removed, usually caused by bond stretching along crystallographic planes in an ordered solid.
Stress
The three types of stress
1. Tension stress: Equal and opposite forces acting away from each other along the same line of action that tend to elongate the body.
2. Compression stress: Equal and opposite forces acting away towards each other along the same line of action that tend to shorten the body.
3. Shear stress: Equal and opposite forces acting along different lines of action that tend to twist the body without changing its volume.
The amount of deformation is proportional to the applied stress only until the applied stress reaches the elastic limit.
Within the elastic limit when the deforming force is removed the body returns to its original shape and volume.
At some stage applied stress beyond the elastic limit the body can no longer be deformed and so it breaks.
For example, stretch springs of copper and brass.
The copper spring remains extended, because it has reached its elastic limit.
Shearing causes shear strain, where parallel surfaces slide past one another, occurs when forces are applied to shears and scissors.
34.8.9 Viscoelasticity, creep
Viscoelastic materials show viscous and elastic characteristics when deformed.
They have the relationship between stress and strain depending on time.
All materials have some viscoelastic response.
The behaviour of steel or aluminium, at room temperature and under small strain, does not deviate much from linear elasticity.
Viscoelastic effects occur in synthetic polymer foams, polystyrene, guitar strings, tuning forks, high temperature metals, shoe insoles, human spinal discs, skin.
For example, if skin is pinched, the harder the pinch the longer it takes to return to its normal position.
Creep is a slow, progressive deformation of a material under constant stress.
Behaviour of viscoelastic materials::
1. If the stress is held constant, the strain increases with time, viscoelastic creep.
2. If the strain is held constant, the stress decreases with time, viscoelastic relaxation.
3. The effective stiffness depends on the rate of application of the load.
4. If cyclic loading is applied, hysteresis, a phase lag, occurs, leading to a dissipation of mechanical energy.
5. Acoustic waves experience attenuation.
6. Rebound of an object following an impact is less than 100%.
7. During rolling, frictional resistance occurs.
34.8.3 Bulk modulus, modulus of incompressibility, K
Bulk modulus, (K or B), shows how much pressure to apply to a material to cause a certain deformation. Compressive stress / Volumetric strain =
Deformed force per unit area / Change in volume per unit volume = K.
so K = (F/A) / (change in volume v / original volume V) = PV /v = K
[Compressibility = 1/K].
Unlike gases, liquids and solids have little space between the particles so are difficult to compress.
Solids are more difficult to compress than liquids.
Some bulk modulus approximate values: stainless steel 160 GPa, glass 35 -55 GPa, water 2.1 GPa (so water is not completely incompressible!).
Water: 2.1 GPa gigapacals = 300,000 psi, (pounds per square inch). Stainless steel approximately 80 times harder to compress than water.
To calculate bulk modulus: B = E / 3(1 - 2ν), where: B = Bulk's modulus, E = Young's modulus, ν = Poisson's ratio.
34.8.11 Young's modulus, E, elasticity, stress and strain
See diagram 35 5.04: Young's modulus.
See diagram 35 5.05: Modulus of rigidity apparatus.
Strain is a force tending to pull a something apart, or push a something against a resistance, or alter the shape of something.
Young's modulus describes the proportional deformation produced in something by the application of a stress.
Strain has no dimensions.
Stress is the pressure or tension exerted on something that tends to deform it.
Stress is measured in units of pressure.
(Linear stress / Linear strain) = (Deforming force per unit area / Change in length per unit area) = (F/A) / (increase in length e / original length L) = FL/eA = E.
The SI unit of Young's modulus is the pascal, Pa.
However, in practice, the common unit is gigapascals, GPa, i.e. zzz The Bulk Modulus Elasticity - or Volume Modulus - is a measure of the substance's resistance to uniform compression. Bulk Modulus of Elasticity is the ratio of stress to change in volume of a material subjected to axial loading. Metals and Alloys - Bulk Modulus Elasticity Material Bulk Modulus - K - (106 psi) (GPa) Aluminum, various alloys 9.9 - 10.2 68 - 70 Brass, 70-30 15.7 108 Brass, cast 16.8 116 Copper 17.9 123 Iron, cast 8.4 - 15.5 58 - 107 Iron, malleable 17.2 119 Magnesium alloy 4.8 33.1 Monel metal 22.5 155 Phosphor bronze 16.3 112 Stainless Steels 18-8 23.6 163 Steel, cast 20.2 139 Steel, cold rolled 23.1 159 Steel, various 22.6 - 24.0 156 - 165 1 GPa = 109 Pa (N/m2) Stainless steel with Bulk Modulus 163 109 Pa is approximate 80 times harder to compress than water with Bulk Modulus 2.15 109 Pa. Bulk Modulus is related to Modulus of Elasticity and Poisson's Ratio as K = E / 3 (1 - 2 r) (1) where K = Bulk Modulus (Pa (N/m2), psi (lbf/in2) E = Modulus of Elasticity (Pa (N/m2), psi (lbf/in2) r = Poisson's Ratio zzz
34.8.11 Young's modulus, E, elasticity, stress and strain
See diagram 35 5.04: Young's modulus.
See diagram 35 5.05: Modulus of rigidity apparatus.
Strain is a force tending to pull a something apart, or push a something against a resistance, or alter the shape of something.
Young's modulus describes the proportional deformation produced in something by the application of a stress.
Strain has no dimensions.
Stress is the pressure or tension exerted on something that tends to deform it.
Stress is measured in units of pressure.
(Linear stress / Linear strain) = (Deforming force per unit area / Change in length per unit area) = (F/A) / (increase in length e / original length L) = FL/eA = E.
The SI unit of Young's modulus is the pascal, Pa.
However, in practice, the common unit is gigapascals, GPa, i.e. kN / mm2.
(In USA, pounds force per square inch (PSI), but in practice ksi, i.e. thousands of pascals per square inch.).
Some Young's modulus values: | steel 200 GPa | glass 65 GPa | aluminium 70 GPa | polystyrene 3 GPa|.
The Young's modulus values of different types of chemical bonds can be measured:
Covalent bonds, e.g. C-C bonds, 200-1000 GPa,
Metallic bonds, e.g. all metals, 60-300 GPa,
Ionic bonds, e.g. Alumina, Al2O3, 32-96 GPa,
Hydrogen bonds, e.g. Polyethylene, 2-12 GPa,
Van der Wall's bonds, e.g. waxes, 1-4 GPa.
Experiment
1. To show that stress is proportional to strain in a wire under load.
Suspend two wires of the same material, parallel from the same support.
The scale on the first wire is kept taut an attached weight.
Extend the second wire with different loads and measure the extension witha vernier scale attached to the first wire.
Measure the unextended length and the diameter of the wire with a rule and a micrometer.
Calculate Young's modulus from the gradient of the graph and the measured data.
2. Young's modulus of students
Dr Richard Walding collected the following data in a Brisbane high school using Year 7 girls
Aim: To measure the body lengths lying on the floor and standing upright to see the effect of gravity.
The average shrinkage when standing was 2.4 cm.
Class averages:
Length lying 161.2 cm
Length standing 158.8 cm
Delta L = 2.4 cm
Waist circumference 65.8 cm
Mass 37.2 kg
Calculations:
Force (weight) 365 N
Cross sectional area = 0.034 sq m Tensile strength (F/A) = 10621pa
Pa Strain (delta L/L) = 0.0152
Young's Modulus = 0.700 MPa
(Young's Modulus for biological tissue = 0.2 MPa).
34.8.7 Shear modulus, modulus of rigidity, G
1. Shearing stress /Shear strain = (F/A) / change in an angle of π/2 radians(90oC)
If forces are applied tangentially to the upper and lower surfaces of a cube causing the shape to change without change in volume, section of the.
cube at right angles to those two faces will have their angles changed from π/2 to (π/2 + θ) or (π/2 - θ).
Young's modulus is related to shear modulus, G, Poisson's ratio v, and bulk modulus, K, by the formula:
E = 2G(1+ v) = 3K(1-2v) = 9KG / (3K + G).
2. The apparatus consists of a rod clamped at one end and attached to a wheel at the other.
The rod passes through a bearing at the wheel end and known torque may be applied by a string wrapped around the wheel.
The twist in the rod is measured with an angular scale.
3. Solids have modulus K, modulus E and modulus G.
Liquids have modulus E and modulus K only.
Gases have modulus K only.
Some shear modulus values at room temperature: steel 79 GPa, glass 26 GPa, aluminium 25 GPa, polyethylene 0.117 GPa.
34.8.6 Poisson's ratio, v
A longitudinal pull in one direction produces an extension in that direction and a contraction at right angles to that direction, the stretched body becomes thinner.
The ratio of the lateral contraction per unit breadth to the longitudinal extension per unit length in the line of applied force is the Poisson's ratio for the material, v.
Experiments
1. Stretch a rubber hose to show lateral contraction with increasing length.
2. Use a two dimensional spring model to show Poisson contraction in crystals.
34.8.5 Hooke's Law, elastic limit, deforming force, stress and strain
See diagram 34.5.1: Young's modulus, Poisson's ratio, Bulk modulus, Shear modulus
See diagram 34 5.1.1: Hooke's law, spring.
Materials that recover their original shape after an applied force is removed, show elastic deformation.
Materials that do not recover their shape after an applied force is removed, show plastic deformation, because the applied force was greater than elastic limit.
Stress is the applied force per unit area of a material.
Stress may cause a strain.
Strain is the change in dimensions of a material / original dimensions of the material, e.g. change in volume per unit volume.
Hooke's law states that, within the elastic limit, the stress is proportional to the strain.
The constant of proportionality, elastic constant, for a material is called Young's modulus, E.
With wires made of iron or annealed steels, at the elastic limit (yield point), a sudden plastic deformation occurs.
The wire "gives" and despite decrease of stress, the wire does not return to its previous shorter length.
Hooke's law does not apply to polymers or rubber.
When a small stress results in a big strain, the material is soft.
When a big stress results in a small strain, the material is hard.
When a small stress results in permanent deformation, the material is plastic.
A modulus is a numerical quantity representing some quality of a substance equal to the ratio of magnitude of the cause to magnitude of effect on the substance.
The bulk modulus of a material is often expressed for convenience in GPa, gigapascals.
1 gigapascal = 1000000000 pascal, 109 pa.
Experiment
1. Mark the beginning and end positions of several different masses.
Compare the end positions of masses that are multiples, such as double or triple.
2. With the spring held in the pin vice on the stand, weights are added in steps of 1 kg.
The greater the weight added, the greater the extension of the spring and the relation between them should be linear in nature according to Hooke's Law.
Releasing the weights should see a return to the original extension.
When this is occurring the spring is displaying elastic deformation.
If enough weight is added to the spring, it will deform permanently and lose its elasticity.
This is known as plastic deformation.
It is interesting to note that if the experiment is repeated with rubber bands, Hooke's Law is not followed.
Rubber is extremely non-linear in its stress-strain characteristics.
3. Pull on a horizontal spring with a spring scale.
Use two metres of copper wire, e.g. 32 SWG, stretched by weights attached to the end the wire through a pulley.
Plot a graph of load against extension of the wire.
The graph is a straight line to show that Hooke's law applies, extension is proportional to stretching force.
Take off weights and observe that the wire returns to its previous lengths at the same tensions.
4. Repeat the experiment by adding weights until the wire suddenly "gives" or "runs".
This is called the yield point.
The wire has stretch proportionally much more than previously for the load added.
The wire can support heavier loads.
However, when the weights are removed, the wire can no longer return to its original lengths.
At the yield point the wire had reached its elastic limit and Hooke's law no longer applies.
In engineering, metal components should carry loads only within their elasticlimits.
34.8.20 Use springs
Add masses to a pan balance and measure the deflection with a vernier scale or cathetometer (travelling microscope).
Examine the force / displacement curve at small extensions.
Add 10, 20 and 30 newton to a large spring.
34.11.2 Breaking strains, brittleness
Brittle A material distorted by forces acting on it is in a state of strain, is strained.
So strain is the ratio: change in dimension / original dimension., and hasno units.
Direct tensile or compressive strain = elongation or contraction / original length.
Shear strain causes a rectangle to become a parallelogram.
Volumetric strain, bulk strain = change in volume / original volume.
Approximate breaking strain in kg of some metals and wires hard drawn through the same gauge (No. 23):
Copper 12 kg, Tin, < 3 kg, Lead < 3 kg, Tin-lead (20% lead) 3 kg, Tin-copper (12% copper) 3 kg, Copper-tin (12% tin) 40 kg.
Gold (12% tin) 9 kg, Gold-copper (8.4% copper) 32 kg,
Silver (8.4% copper) 20 kg, Platinum (8.4% copper) 20 kg, Silver-platinum (30% platinum) 34 kg.
However, the malleability, ductility, and power of resisting oxygen of alloys is generally diminished.
The alloy formed of two brittle metals is always brittle.
The alloys formed of metals having different fusing points are usually malleable while cold and brittle while hot.
The action of the air on alloys is generally less than on their simple metals, unless the former are heated.
A mixture of 1 part of tin and 3 parts of lead is scarcely acted on at common temperatures, but at a red heat it readily takes fire, and continues to burn for some time.
Similarly, a mixture of tin and zinc, when strongly heated, rapidly decomposes both moist air and steam.
Brittleness is the tendency for metals or alloys to have a brittle fracture when under tension, without plastic deformation, i.e. still keeping its shape.
Brittleness mean a low value of fracture toughness, toughness.
Brittle fracture is causes by cracks leading to more cracks usually along certain crystal planes.
34.15.3 Breaking threads
1. Place a broom handle across two stools.
Attach a thread to be tested to the centre of the broom handle.
Attach the lower end of the thread to a large plastic bottle.
Add water to the jar until the thread breaks.
Note the volume of water needed to break the thread.
2. Add heavy masses to different threads until they break, e.g. cotton thread, thin copper wire, fishing line, dental floss, wool yarn, catgut, piano wire.
Compare the breaking strain of the fishing line with this information on the packet.
34.15.2 Breaking spaghetti
Hold each end of a length of dry spaghetti with two hands.
Bring the hands together to bend the spaghetti until it snaps.
The spaghetti always breaks into more than two pieces, usually three pieces.
The spaghetti breaks when when the amount of curvature approaches a critical value called the rupture curvature.
The broken ends straighten sending waves of curvature back towards the hand which, in spaghetti can interact to cause another break.
34.15.6 Shear strength of thin sheets
See diagram 34.5.2.3: Clothes-peg tester.
Cut sheets of material to be tested so that they just fit around a spring clothes peg, e.g. newspaper, paper towel, potato chip packet, thin plastic, cling film.
All the sheets should have the same shape and area.
Wrap each sheet around the spring clothes peg and squeeze the ends of the clothes peg handles with the thumb and first finger.
Note which materials stretch or break.
34.15.1 Bend beams, bend metre stick, stress, rectangular bar, woods
Observe bending beams
Bend beams, bend metre stick, stress rectangular bar, different woods
Hang 2 kg from the centre of a metre stick supported at the ends.
Place the metre stick on edge and then on the flat bending beam.
Load a rectangular cross-section bar in the middle while resting on narrow and broad faces.
Hang weights at the ends of extended beams.
Use beams of different lengths and cross sections.
Use different woods.
34.15.5 Sagging board, aluminium / steel elasticity paradox
Sagging board, aluminium / steel elasticity paradox
Show that copper and brass rods sag by different amounts under their own weight, but steel and aluminium do not.
34.15.7 Use strain gauge
Stretch a hole, deformation under stress, stress on a brass ring
See 2.05: Conic sections, ellipse.
Stretch holes arranged a circle in a rubber sheet to deform into an ellipse.
Paint a pattern on a sheet of rubber and deform by pulling on opposite sides.
Use a strain gauge bridge to measure the forces required to deform a brass ring.
34.15.8 Use a squeeze bottle
Fit a bottle with a stopper and a small bore tube.
Squeeze the bottle and watch the coloured water rise in the tube.
34.15.4 Prince Rupert's drops, tempered glass, toughened glass
Bubbles made by dropping molten glass into water.
The shape is like that of a tadpole.
If the smallest portion of the end of the tail is nipped off, the whole bubble explodes into fine dust.
This novelty was introduced into England by Prince Rupert (1619-682), grandson of James I.
He also introduced Prince Rupert's metal, an alloy of brass.
Experiment
Cool a drop of molten glass very quickly.
Hit the round bulb of the glass with a hammer.
It does not break.
Break off the sharp tip of the drop.
The glass shatters.
This is a form of tempered glass (toughened glass), manufactured into sheets that break into small granular chunks, instead of dangerous pointed shards.
The sheets are used in buildings, telephone box windows and the side windows of cars.
Car windscreens are made of laminated toughened glass.
Another method of producing toughened glass for complex shapes, e.g. drinking glasses, involves treating glass in molten potassium nitrate.
e.g. For sale: Tumbler, toughened glass, 230 mL, pack / 6.
34.15.10 Use tennis balls
A tennis ball should bounce to a height of 135 to 147 cm when dropped on a concrete floor from a height of 254 cm.
It should reach between 53% and 57% of the drop height when bouncing.
If not, use tennis balls to protect from sharp corners of furniture, grip strengthener, rolling foot massage, back massager, hit distant cobwebs, prevent chairs slipping.
34.8.14 Use coffee, coffee tins
Use coffee to dye fabric brown, fertilize houseplants, repair scratched woodwork, deter ants.
Use coffee cans with lid, coffee tins, to spread grass seed, protect baby tomato plants, raise melons off the ground, demonstrate expansion of heated gases.
34.8.17 Use lip balm
Use lip balm as protectant and lubricant on skin, skin cuts, car battery terminals, zippers, ring fingers, nails and screws, leather shoes, drawers and windows.
34.8.18 Use nail polish
Use nail polish to stop cut fabric fraying and runs in nylons, repair cracks, prevent rust on bottom edges of cans, thread needles, protect shiny surfaces and labels.
Acetone was formerly known as "nail polish remover".
34.15.14 Use toothpaste
Use white toothpaste to polish silverware, clean small objects, remove ink spots, treat facial pimples, remove crayon marks and scratches, fill small holes in walls.
Do not use gel toothpaste for these jobs.
34.15.15 Use toothpicks
Use toothpicks to apply glue, plug holes, draw designs in sand, suspend seed potato in water, repair eyeglass lugs, mark starting point of a tape roll, clean tight spaces, small plant splints, turn sausages, tighten a loose screw, push fabrics through pressure foot of a sewing machine, test if cake is baked.
34.13.0 Shear stress
Structures Tester, stress and strain, (Commercial).
Shear is deformation of materials where parallel plates of the material are displaced in a direction parallel to themselves, but the parallel plates remain parallel.
So the adjacent planes of parallel plates slide over each other.
If a shearing force is applied parallel to one side of a rectangle it becomes a parallelogram.
Shear stress is the applied force divided by the area of the material parallel to the applied force, i.e. F / 1.
Experiment
Use scissors to cut 1. a sheet of paper, 2. tough plastic tube.
The paper is cut sharply, but the plastic tube stretches between the blades of the scissors.
34.13.1 Shear books, foam block
Experiment
Use a very thick book or stacks of cards to show shear.
Push on the top of a large book or a large foam block to show shear.
34.13.3 Plywood sheets, shear torsion
Experiment
Use a stack of plywood sheets with springs at the corners to show shear torsion bending.
34.13.4 Torsion rod, modulus of rigidity, bending and twisting
Experiments
1. Twist a rod by a mass hanging off the edge of a wheel.
2. Wind a copper strip around a rod and then remove the rod and pull the strip straight to show twisting bending and twisting.
3. Twist rods of various materials and diameters in a torsion lathe.
4. Grab each end of a plastic ruler and twist the ends in opposite directions.
34.1.1 Test the strength of a simple bridge
Experiments
See diagram 34.6.2: Simple beam bridge.
See diagram 34.6.2: Simple beam bridge.
1. Use C-clamps and blocks of wood to fix one piece of wood, e.g. 0.5 cm × 5 cm × 60 cm, between tables 0.5 m apart.
Use rope to attach an empty bucket to the centre of the bridge.
Add sand to the empty bucket until the wood bends downward 1 cm at the centre.
Weigh the bucket and sand.
Add sand until the wood breaks.
If the wood does not break just use the data: Weight to bend down 1 cm.
2. Decrease and increase the distance between the tables to find the weight needed to bend the wood downward 1 cm and the weight to break the wood.
The piece of wood must be free of knots.
3. Repeat the experiment with the narrower width (0.5 cm) down (vertical board).
4. Drill two holes vertically in the board and repeat the above experiments.
Table 34.6.2
Distance between tables Weight to bend down 1 cm Weight to break wood
50 cm .
 .
55 cm .
 .
45 cm .
 .
34.8.13 Strength of paper, shape and its mechanical strength
See diagram 34.3.1: Folded paper, crossbeams.
A flat piece of paper placed over two rods can support only light weight.
However, if the piece of paper is folded into many alternate ditches and edges it can support heavier weight.
Experiment
Draw parallel lines on A4 paper 1 cm apart.
Fold the paper alternately each way along the parallel lines.
Cut out a 4 cm square of cardboard and put it on folded paper.
Add weights to the cardboard or put an empty glass on it and add water until the paper begin to change its shape.
Repeat the experiment with paper folds 0.5 cm apart and 2.0 cm apart.
Compare the results of the two experiments.
Crossbeams made of reinforced concrete are used in building construction as in diagram 34.3.1.(b), not as in diagram 34.3.1.
34.4.1 Bouncing balls, Silly putty, silicone, bouncing putty, "Tricky Putty"
Experiments
Drop balls of different material on plates of various materials.
Observe loss of mechanical energy in the coefficient of restitution.
Drop balls on a glass plate.
Drop glass, steel, rubber, brass, and lead balls onto a steel plate.
Drop rubber balls of differing elasticity and silly putty on a steel plate.
Observe variation in coefficient of restitution n baseballs.
(Dow Corning 3179 dilatant compound).
34.4.2 Coefficient of restitution (coefficient of elasticity)
See diagram 34.7.2: "Happy ball bounces, sad ball does not bounce."
Elasticity
If a ball mass m is dropped from height h1 and rebounds to height h2, the loss of energy = mg(h1-h2).
The energy loss is expressed as the coefficient of restitution, e = v2/v1 = sqrt h2/h1, where v1 is the incident speed and v2 is the rebound speed
Experiments
1. Drop bounce and no-bounce balls.
Measure the height the bouncing ball is dropped from, and the height it bounces to, and calculate the coefficient of restitution.
The "sad" ball will not bounce as it is made from energy absorbing material.
2. Newton found experimentally that if two smooth spheres collide with velocities u1 and u2 and rebound with velocities v1and v2,
then - (v2 - v1) / (u2 - u1) is a positive constant, e, independent of the initial velocities, called the coefficient of elasticity or coefficient off restitution.
The value of the constant e depends on the substances, e.g. 0.9 for glass and 0.2 for lead.
3. The coefficient of restitution can be used to measure of the elasticity of the collision between ball and racquet.
Elasticity is a measure of bounce, i.e. how much of the kinetic energy of the colliding objects remains after the collision.
With an inelastic collision, some kinetic energy is transformed into deformation of the material, heat, sound, and not available for movement.
For a perfectly elastic collision, coefficient of restitution = 1, e.g. two diamonds colliding.
For a perfectly plastic, i.e. inelastic, collision, coefficient of restitution = 1, e.g. two lumps of Plasticine (modelling clay) that do not bounce, but stick together.
The coefficient of restitution = difference in velocities of two colliding objects after the collision / difference in velocities of two colliding objects after the collision.
For a racquet and ball, v1 = velocity racquet centre before impact, s1 =velocity ball before impact, v2 = velocity racquet centre after impact, s2 = velocity ball after impact.
Coefficient of Restitution = (s2 - v2) / (v1 - s1)
For a falling object bouncing off the floor, coefficient of restitution = (bounce height / drop height), e.g. for a particular bouncing ball, coefficient of restitution = 0.85.
34.4.3 Dead and live balls
Experiment
1. Drop a black super ball and a ball rolled from a piece of wax.
2. Make a non-bounce ball by filling a hollow sphere with iron filings or tungsten powder.
34.8.16 Use ice
Ice model (Commercial)
Experiment
1. Make ball and stick water molecules that you can stick together to make ice.
2. Let large ice crystals form on the surface of a super cooled saturated sugar solution.
34.8.15 Use crystals
1. Observe crystal growth on a freezing soap film through crossed Polaroid.
2. Arrange one layer of small ball bearings between two Lucite (Perspex), sides.
3. Examine natural faults in a calcite crystal then the single layer of small spheres model faults.
4. Crush a large salt crystal in a big clamp.
34.8.19 Use nitinol memory wire
"Nitinol Memory Wire", Ni Ti alloys, shape memory alloy (toy product).
Convert thermal energy into mechanical energy with a shape memory alloy, Nitinol.
The thermobile consists of a shape memory alloy nitinol (Nickel-Titanium).
Above a certain temperature, Tc, this alloy returns to an earlier shape given it by previous heat treatment.
The alloy absorbs heat as it returns to this shape converting it into mechanical work.
Dip the small metal wheel of the thermobile into hot water to raise the temperature of the alloy above Tc and rotation follows.
34.8.12 Test solid models, sphere packing
Use tetrahedral and octahedral building blocks construct crystal shapes.
Use Styrofoam balls and steel ball bearings to make crystal models.
Stack balls on vertical rods mounted on a board to build crystal models.
Build crystal models with a combination of compression and tension springs.
Use old tennis balls glued together to show close-packed crystals.
Examine lattice models of sodium chloride, calcium carbonate, graphite and diamond.
34.6.2 Noise sources, test A
Use a knock-down [be able to be dismantled] transformer.
Install its primary coil and secondary coil well and let its iron core in not closed state (viz. do not install the upper iron frame).
Turn on the AC electrical source for the primary coil and observe the vibrationand sound of the iron core.
Make the iron core closed, but do not screw the screws tightly and note the change in sound.
Screw the screws tightly.
You may find noise lowers observably.
Many noises are caused by disordered vibration of some components without being fixed well.
Be careful not to touch the metal parts of the transformer, because it carries AC of more than 36V.
Place a plastic ruler on a tabletop flat and let it spread 1/3 long out of the table and vertical to the table rim.
Press the end at the table with your left hand and take a press on another one with your right to make the ruler vibrate.
Note the vibration on the tabletop and the noise it emits.
Place a large, thin, sponge pad under the ruler to separate the ruler and the table.
Repeat the above experiment.
You may hear only the sound the ruler vibrates.
Adding some elasticity materials under vibrating objects may lower vibration noise effectively, because elasticity materials may absorb vibration energy.
34.6.3 Noise control, test B
Use a small radio and a box Turn on the radio to the most volume.
Place the radio into the box then cover its cap.
Listen to the sound.
You may find the sound decreases slightly.
Separately put some cotton, sponge and broken stones in the space between the radio and box wall.
Listen to the sound again.
You may find cotton and sponge make the sound decrease more observably.
Actually many spongy materials are sound absorption materials.
If place them at the places transferring noise, they can lower noise effectively.
34.6.4 Pollution from light of buildings
Many modern buildings' outside walls are decorated with glass mirrors.
Thus there is much sunlight being reflected to fixed direction.
The inhabitants living at the places opposite to the buildings are under the strong light pollution.
For example, their rooms are hotter in summer, their children's eyesight lowers due to the strong light's stimulation.
Experiment
To study how reflected sunlight makes the temperature at a small space increase in summer obtain two same large boxes.
For paper boxes, wrap a layer of thin heat insulation materials such as foam sponge and cotton pad to imitate the walls of a room.
Cut a window at a side of each box, making sure the two windows with the same size.
Shade the windows with transparent glass paper or plastic film.
Place the boxes in the sunlight in summer, but without sunlight shining in the boxes directly.
Insert a thermometer into each box.
Place a large mirror and adjust its position to make reflected sunlight into a box through its "window".
You may find the temperature at the box shined on by reflected sunlight increases quickly.
Carefully note the difference in temperature of the two "rooms" until the temperature at this box increases no longer.
Record the readings of the temperatures and calculate the difference in temperature between two boxes.
Remove the transparent glass paper shading each window to imitate "opening windows to air".
After a while, you may find the temperature at the box shined by reflected sunlight decreases more slowly than another box.
Carefully note the difference in temperature of the two "rooms" until the temperature at each box decreases no longer.
Record the readings of the temperatures and calculate the difference in temperature between two boxes.
34.6.5 Pollution from noise, noise effects thinking and learning, white noise
Often people use the word "sound" for something they want to hear, and "noise" for what they do not want to hear.
In general, musical sounds are made up of a certain limited number of frequencies.
They are regarded as sounds even though some people may not want to hear them.
Motor traffic, aircraft and trains all produce a complex range of sounds of many unrelated frequencies at the same time.
This is described as noise.
It is a random mixture of sounds of different frequencies and amplitudes.
Study the reasons causing noise and the ways lowering noise.
34..1 Electrostatic precipitation
See diagram: 34.4.3: Precipitators.
To build a model to show the action of an electrostatic precipitator you need concentrated hydrochloric acid, concentrated ammonia solution, gas jar or measuring cylinder, test-tubes, thin metal rod, glass and plastic tubing, stoppers, induction coil and leads, aquarium pump and aluminium foil. The aluminium foil making up the outer electrode should be in the form of a cylinder inside the walls of the jar, but if you want to see what is happening inside, you may leave a space.
Turn on the pump.
Hydrogen chloride from the acid reacts with ammonia from the next test-tube to form a smoke of ammonium chloride.
Notice the amount of smoke emerging from the chimney.
Gradually increase the flow of air from the pump then turn on the induction coil to supply the high voltage.
Note any change in the smoke from the chimney.
34.10 Hardness, Mohs scale of mineral hardness
Minerals differ greatly in hardness.
The hardness refers to the resistance of a mineral to scratching, scratch hardness.
The Mohs scale of hardness (Friedrich Mohs, Germany, 1773 - 1839) has a range from 1 (softest) to 10 (hardest).
Hold a specimen of a mineral with forceps and try to scratch the following substances with it:
fingernail hardness 2.5
piece of copper or copper coin hardness 3
steel knife blade hardness 3.5
window glass hardness 3.5 to 6.0
steel file hardness 6
diamond hardness 10
American coins have hardness 2.5, but the old "Indian heads" penny has hardness 3.5.
Hardness 7 substances produce sparks when hit with steel.
Be careful! When hardness testing with glass, do not hold the glass in the hand, but place it on a flat surface.
Mohs scale of hardness of minerals: 1. talc 2. gypsum 3. calcite 4. fluorite 5. apatite 6. orthoclase feldspar 7. quartz 8. topaz 9. corunum 10. diamond
The Mohs scale of hardness of gemstones: topaz 7 emerald 8 sapphire 9 ruby 9 diamond 10.
An unglazed porcelain streak plate used by geologists has approximately 7.0 hardness
This hardness test can be applied only to fresh unweathered specimens.
The columnar mineral kyanite is unusual, because has hardness 4-4.5 vertically, but hardness 6-7 horizontally.
Fibrous and porous aggregates may have a deceptively lower hardness, because of the spaces between grains.
Determining the hardness of earthy minerals, fine grain minerals and needle-shaped fibrous minerals is almost impossible.
Engineers do not use Mohs scale, but define harness as resistance to indentation by a tool tipped with a pyramid-shaped diamond.
The scales include "Vickers", "Rockwell" and "Knoop", in units of force (newton) / diameter2 of the indentation, at an angle of 136o.
For example, the Australian "kangaroo" $1 Aluminium Bronze coin blanks have Vickers hardness 80.
35.5.0 Elements in the Earth's crust
Elements can combine to form natural compounds called minerals.
For example, oxygen and silicon combine to form silica SiO2 that occurs as the common mineral quartz.
Table 36.3.01 Elements in the Earth's crust
Element % Mass Element % Mass
Oxygen 46.71 Carbon 0.094
Silicon 27.69 Manganese 0.09
Aluminium 8.07
 Barium 0.05
Iron 35.05
 Sulfur 0.052
Calcium 3.65
 Chlorine 0.045
Sodium 2.75 Nitrogen 0.03
Potassium 2.58
 Chromium 0.035
Magnesium 2.08
 Fluorine 0.029
Titanium 0.62 Zirconium 0.025
Hydrogen gas
 0.14 Nickel 0.019
Phosphorus 0.13 all other elements 0.061
Many different versions exist of tables to show the most abundant elements in the Earth's crust, for example:
Let "most abundant" refer to % of total mass and not the number individual atoms.
On the earth's crust the most abundant element is oxygen, (about 46 %), because oxygen is a very common rock-forming element, together with silicon (28 %).
Other common elements are aluminium (8.2%), iron (5.6%), calcium (4.2%), sodium (2.5%), magnesium (2.4%), potassium, (2.0%).
So aluminium is the most common metal in the earth's crust, which makes up only a tiny portion of the entire earth.
Beneath the earth's crust, the mantle contains 44.8% oxygen, 23% magnesium and 22% silicon, and the mantle makes up about 84% of the earth's volume.
However, the closer to the earth's core the more dense the Earth.
In the core, gravity is so strong the electromagnetic force between atoms is overcome, allowing for fusion fission to occur and the most "stable" form for nuclei is in the form of iron.
The binding energy per nucleon in the nucleus is the highest in iron-56.
Any element to the left of iron will go through fusion to create iron and every element to the right will go through fission and eventually become iron.
So the most abundant element in the Earth is iron at 32.1%, then oxygen at 30.1%, then silicon at 15.1%, and magnesium at 13.9%.
35.6.0 Geological time scales
Table 35.6.0 Australian Museum Geological Divisions (edited)
Era
 Period
 Epoch
 Time million years ago
Precambrian
 Precambrian
 .
 > 545
Palaeozoic
 Cambrian
 .
 545 - 490
.
 Ordovician
 .
 490 - 434
.
 Silurian
 .
 434 - 410
.
 Devonian
 .
 410 - 354
.
 Carboniferous
 .
 354 - 298
.
 Permian
 .
 298 - 251
Mesozoic
 Triassic
 .
 251 - 205
.
 Jurassic
 .
 205 - 141
.
 Cretaceous
 .
 141 - 65
Cenozoic
 .
 Palaeocene
 65 - 55
.
 .
 Eocene
 55 - 38
.
 .
 Oligocene
 38 - 23.3
.
 .
 Miocene
 23.3 - 5
.
 .
 Pliocene
 5 -1.6
.
 Quaternary
 Pleistocene
 1.6 million -10, 000 years
.
 .
 Holocene
 10, 000 years - to present