Types of materials
- Woods: from trees. Hardwoods (teak, oak) and softwoods (pine). Manufactured boards: plywood, MDF.
- Metals: iron, steel, aluminium, copper. Strong, shiny, good conductors. Mixtures of metals are alloys (steel, brass).
- Plastics (polymers): thermoplastics can be re-melted (PET bottles); thermosets set hard for ever (switch covers).
- Glass and ceramics: hard, heat-resistant, but brittle.
- Textiles: fabrics made from fibres.
- Composites: two materials together for the best of both (reinforced concrete, fibreglass, carbon fibre).
Properties of materials
- Strength: how much force before it breaks.
- Stiffness: how little it bends. Opposite: flexibility.
- Hardness: resists scratches and dents.
- Toughness: takes a knock without cracking. Opposite: brittle (glass).
- Density = mass ÷ volume (g/cm³). Less than water (1 g/cm³) floats.
- Thermal conductivity: carries heat. Metals are conductors; wood, plastic, air are insulators.
- Electrical conductivity: copper wires, plastic cover.
- Others: transparency, water resistance, corrosion (rusting), cost, recyclability.
Engineers also study materials for energy: light alloys for cars, silicon for solar cells, insulation for buildings.
Choosing the right material
Ask in this order:
- Function: what must the product do? (hold weight, carry heat, keep water out)
- Properties needed: list them.
- Making: can we cut, bend, mould or join it with our tools?
- Cost and availability.
- Look and feel (aesthetics), safety for the user.
- Environment: renewable? recyclable? how much energy to make?
Example: a school chair seat — plywood or polypropylene (strong, light, cheap, easy to clean); frame — steel tube (strong, stiff).
Joining methods
- Temporary (can be undone): screws, nuts and bolts, clips, knock-down fittings.
- Permanent: nails, glue (PVA for wood, epoxy for many materials), rivets, welding (melts metal pieces together), soldering (low-melting metal joins wires), wood joints (dowel, mortise and tenon).
- Textiles: stitching, seams, fusing with heat, press studs, zips.
Choose a joint by the materials, the strength needed, whether it must come apart, and the look.
Clothing materials: fibres and fabrics
Natural fibres: cotton (cool, absorbs sweat), linen, wool (warm, traps air), silk (smooth, shiny). Synthetic fibres: polyester, nylon, acrylic — strong, dry fast, crease less, but do not absorb sweat and melt near flames.
Fibres are spun into yarn, then woven (criss-cross, firm) or knitted (loops, stretchy). Blends like poly-cotton mix the best of both. Dyes add colour; natural dyes come from plants such as indigo and turmeric.
History: people first used skins and plant fibres, then spun and wove by hand; machines made cloth cheap in the 1800s; synthetic fibres arrived in the 1900s.
Try it: the kitchen materials hunt
Collect a steel spoon, a wooden spoon and a plastic spoon. Stand them in a cup of hot (not boiling) water for one minute and touch the tops — which is warmest? Then drop each into a bucket of water — which float? Write a table: material, bends?, gets hot?, floats?. Then pick the best spoon for stirring hot dal and explain why. In the 3D, try each job in free play.
Key formulas and definitions
- Density = mass ÷ volume (g/cm³); water = 1 g/cm³
- Density < 1 g/cm³ → floats in water; > 1 → sinks
- Conductor (metals) vs insulator (wood, plastic, air)
- Strong ≠ stiff ≠ hard ≠ tough
- Joints: temporary (screws, bolts) vs permanent (glue, nails, welding)
- Choosing: function → properties → making → cost → looks → environment
Worked examples
1. A block has mass 120 g and volume 200 cm³. Will it float in water?
Density = 120 ÷ 200 = 0.6 g/cm³. This is less than 1, so it floats (like wood).
2. Choose a material for an electric wire and its covering.
Wire: copper (excellent electrical conductor, flexible). Covering: PVC plastic (electrical insulator, flexible, waterproof).
3. Which joining method suits a steel gate, a wooden bookshelf that must be moved often, and a cotton shirt?
Steel gate: welding (permanent, strong). Bookshelf: screws or knock-down fittings (temporary, can be taken apart). Shirt: stitching.
4. Why is a cooking pot steel but its handle plastic or wood?
The pot must carry heat quickly to the food, so a conductor (steel) is used. The handle must stay cool to hold, so an insulator (plastic/wood) is used.
Common mistakes
- Mixing strength and stiffness: a rope is strong but not stiff.
- Thinking hard means tough: glass is hard but brittle.
- Thinking heavy objects always sink: a big wooden log floats because its density is below 1 g/cm³.
- Choosing a material only by looks or price and forgetting function and safety.