D. Structures and Mechanisms · Form and Function

Choosing the Right Materials

D2.6 A1.2 A3.1 A1.4About 45 minutes

Learning goals

I can identify the factors that decide whether a material is suitable for a product or structure.
I can compare common building materials using their properties, cost and environmental impact.
I can test materials fairly and use the results to choose one for a job.

Words to know

material
What something is made of, like wood, steel, concrete or plastic.

property
A feature of a material that can be observed or measured, like strength, flexibility or mass.

tensile strength
How well a material resists being pulled apart (tension).

compressive strength
How well a material resists being crushed (compression).

durability
How well a material lasts without wearing out, rusting, rotting or breaking down.

reinforced concrete
Concrete with steel bars (rebar) inside it. The concrete resists compression and the steel resists tension.

life cycle
All the stages of a material's life: getting the raw materials, making it, using it, and recycling or throwing it away.

Hook: why not a chocolate teapot?

A chocolate teapot is a joke because it would melt the moment you poured in hot water. Its form is fine. Its material is completely wrong for the job.

Engineers, architects, designers and skilled trades workers ask the same question every day: what is the best material for this job?

A wood board, a red brick, a steel bar, a sheet of glass, a concrete block and a white plastic pipe side by side on a workbench. — Each material has different properties: some are strong in compression, some bend without breaking, some let light through. Which would you choose for a bridge, a window or a drain? AI-generated image

Remember from Grade 3

In Grade 3 you chose materials by their properties, such as stiff, flexible, durable, waterproof and insulating. In Grade 7 you will weigh many factors at once, including cost, how the product is made and its effect on the environment.

Factors that decide if a material is suitable

Properties for the job:

strength in compression, tension or both
stiffness or flexibility (should it bend?)
mass (does it need to be light?)
durability: does it resist water, rust, rot, sunlight, salt and freezing and thawing?
hardness (does it resist scratches and dents?)
insulation (does it keep heat in, or keep electricity out?)

Cost: the price of the material and of shaping and installing it.
Availability: can you get enough of it, nearby?
Ease of manufacturing: can it be cut, bent, moulded, welded or joined easily?
Safety and health: does it burn easily? Does it give off harmful fumes? Is it safe for food or for children?
Appearance: colour, texture and style.
Environmental impact: is it renewable? Recyclable? How much energy and pollution does it take to make? This is the material's life cycle.

Material	Useful properties	Weaknesses	Often used for
wood	Strong for its mass; easy to cut and join; renewable if forests are managed well; insulates	Can rot, swell or warp if wet; burns; can be eaten by insects	house frames, decks, furniture, docks
steel	Very strong in tension and compression; can be welded; recyclable	Heavy; rusts unless painted or coated; takes a lot of energy to make	bridges, tall building frames, rebar, cars
concrete	Very strong in compression; can be poured into any shape; durable; fire-resistant	Weak in tension (cracks when stretched or bent); heavy; making cement releases a lot of carbon dioxide	foundations, sidewalks, dams
reinforced concrete	Steel bars inside handle tension; concrete handles compression	Heavy; if the steel rusts, the concrete can crack and break off	bridges, parking garages, tall buildings
aluminum	Light; resists rusting; recyclable	Costs more than steel; less stiff than steel	canoes, ladders, window frames, airplanes
plastic	Light; waterproof; easy to mould; low cost	Most are made from oil or natural gas; many are hard to recycle; can break down in sunlight	pipes, containers, helmets, toys
glass	Lets light through; waterproof; hard	Brittle (shatters); heavy	windows, bottles

Comparing common materials

In a loaded beam the top is squeezed and the bottom is stretched. Concrete handles the compression; steel bars near the bottom handle the tension. That team is reinforced concrete. (The bend is exaggerated.)

Worked example

Let's choose a material

A town is building a footbridge over a creek. Choices: untreated pine, steel, or concrete. The bridge will be outside year-round, with snow, rain and road salt from people's boots. Which is most suitable?

Untreated pine: strong for its mass and cheap, but would rot outdoors and wear quickly. Not durable enough.
Steel: very strong, but rusts with water and salt unless it is painted or coated and kept up.
Concrete (reinforced): strong, durable outdoors, but heavy and its foundations must hold that weight. Salt can slowly damage it, so it is often sealed.
Weigh the factors: durability outdoors is very important. Coated steel or reinforced concrete both work; many footbridges use a steel or concrete structure with a pressure-treated wood or composite deck.

Coated steel or reinforced concrete for the main structure is most suitable; untreated pine is not durable enough outdoors. There is often more than one good answer: engineers compare the factors and choose.

Worked example

Let's design a fair test

Which is strongest in tension: a strip of paper, a strip of plastic grocery bag or a strip of aluminum foil?

Change (independent variable): the material.
Keep the same: strip size (2 cm by 20 cm), how it is held, the cup used to hold the load, how coins are added (one at a time, gently).
Measure (dependent variable): the number of coins the strip holds before it tears.
Repeat 3 times for each material and find the average.

A fair test changes only the material and measures the load before tearing, repeated and averaged.

A fair tension test: change only the material, keep everything else the same, and measure how many coins each strip holds. Repeat 3 times and average.

Just for fun

Tea water is often close to 100°C, and chocolate starts to melt at only about 30°C. The chocolate teapot would not survive even one cup!

Hands-on activity

Material tension test

You need: strips 2 cm wide and 20 cm long of paper, plastic grocery bag, aluminum foil and fabric from an old T-shirt; scissors; tape; a paper cup with a string handle; identical coins or washers; a ruler; a table edge. A grown-up should help with scissors.

Tape the top of a strip to the edge of a table so it hangs down. Tape the cup's string handle to the bottom of the strip.
Add coins one at a time, gently, until the strip tears or stretches so much the cup touches the floor. Record the number.
Repeat 3 times with fresh strips of each material. Find the average.
Also observe: which material stretched before breaking? Which tore suddenly?
Choose the best material for (a) a reusable shopping bag, (b) a kite tail. Explain using your data and at least two other factors (cost, mass, durability, environment).

What to look for: results vary, but fabric and plastic-bag strips usually stretch before failing, while foil and paper tear suddenly. A good answer uses data plus other factors, for example: "fabric for a reusable bag because it held the most coins, lasts many uses and can be washed." Keep feet clear of falling coins.

Your turn

Name four factors to consider when choosing a material.

Why is steel put inside concrete to make reinforced concrete?

Why are canoes often made of aluminum?

What is one weakness of wood used outdoors?

What does durability mean?

Name one environmental factor to think about when choosing a material.

More practice: warm-up

Which material is very strong in compression but weak in tension?

Which material is brittle and lets light through?

Which material can rust?

Is wood renewable?

Most plastics are made from what?

More practice: on your own

Why are hockey helmets made of hard plastic with foam inside, instead of steel?

A cottage dock sits in a lake. Compare cedar wood and untreated pine for the deck.

Why might a builder in a remote northern community choose a material that is light and easy to ship, even if it costs more?

Explain why aluminum ladders are popular, and one safety concern.

In the tension test, why must the strips all be the same size?

Challenge

Design a playground slide for a park. Choose a material for (a) the slide surface and (b) the support frame. Justify each using at least three factors.

A construction company says, "Concrete is cheap and strong, so it is always the best choice." Give two reasons this is not always true.

Explain how a material's life cycle could change which material is the best choice for a product.

Check your understanding

Quiz

Which material is used in reinforced concrete to resist tension?

Which is a property of a material?

Why is aluminum a good choice for a canoe?

Which factor is about the environment?

In a fair test of material strength, what should change?

Why is untreated pine a poor choice for a dock in a lake?

For parents and teachers

Big idea (D2.6): Students identify the factors that make materials suitable: properties, cost, availability, ease of manufacturing, safety, appearance and environmental impact. They plan a fair test (A1.2) and connect to the work of skilled trades and engineers (A3.1).

How to help: At a hardware store or around the house, ask "Why is this made of that?" (a metal pot with a plastic handle, a wooden deck, a vinyl window frame). Encourage your child to name at least three factors.

Common difficulties:

Choosing only by strength. Remind your child that cost, durability, safety and environment matter too.
Unfair tests. All strips must be the same size and the load added the same way.
Safety: supervise scissors; keep feet clear of falling coins (A1.4).