Materials: Properties, Sustainability and Justifying Choices

Tensile strength.

Balsa is botanically a hardwood (it comes from a broad-leaved tree), yet it is very soft. The names describe the tree type, not hardness.

Metal has high thermal conductivity to heat food; plastic or wood handles have low thermal conductivity so they stay cool enough to hold.

The pine block, because its density is less than water's (1 g/cm³).

Raw materials, processing, manufacturing, transportation, use (and repair), end of life (reuse, recycling, composting or landfill).

It replaces many disposable items over its life, so less total material and waste; it can often be repaired; fewer trips to manufacture and ship replacements.

A plastic that softens when heated and can be reshaped (often recyclable). Examples: HDPE (milk jugs), acrylic, PET (water bottles).

Mass per unit volume, e.g., g/cm³.

Ferrous (contains iron).

Wood from a well-managed forest; also bamboo, wool, cotton.

Rubber.

How easy a material is to cut, shape, join and finish.

Volume = 785 ÷ 7.85 = 100 cm³. Aluminum mass = 100 × 2.7 = 270 g.

Cedar wood naturally resists rot and insects better than many woods, so it is durable outdoors (water resistance/durability).

Less transportation energy (shorter distance), fresher food with less spoilage and waste; supports local farms.

Water weakens paper fibres and dissolves some glues, so the cardboard loses stiffness and strength. Improve with a water-resistant coating (such as paint or a sealant) or keep it as a prototype only and make the final from wood.

Sample: plastic (a durable, food-safe type): lower density (lighter to carry) and tougher (does not shatter when dropped). Glass is harder and easy to clean but can break, which is a safety concern at school.

Paper is renewable and recyclable or compostable, but it can take more energy and water to make and is heavier to ship. Plastic bags use fossil fuels and can litter waterways, but are light. A sturdy reusable bag used many times usually has the lowest impact per trip. The answer depends on how many times each bag is used and how it is disposed of.

Sample: aluminum has about a third of steel's density, so the frame is lighter for carrying up stairs, and it does not rust the way steel does in salty winter slush. Steel is cheaper, more comfortable over bumps and easy to repair by welding. For a light, low-maintenance commuter: aluminum; for a tough, cheap, repairable bike: steel. Any justified answer with two properties is fine.

Wood is renewable and stores carbon, but only if it comes from well-managed forests; it can be treated with harmful chemicals; and it may be shipped long distances. Some plastics last decades and are recyclable. The more sustainable choice depends on the whole life cycle: source, processing, transport, how long it lasts and what happens at the end. The claim is too simple.