Science 7 · Structures and forces
A structure has one job: hold its shape under load
A bridge, a chair, a bird's nest and your skeleton all do the same job. They keep their shape while forces push, pull and twist them. This unit is about the kinds of structures, the forces they must survive, and the tricks — triangles, arches, folds — that make them strong without making them heavy.
- 1. The words, first
- 2. Kinds of structures and what they are made of
- 3. Forces on a structure and in it
- 4. Stability, strength and good design
- 5. What costs marks
The words, first
The idea: Mass and weight are not the same, and weight is measured in newtons, never kilograms.
| Word | What it means |
|---|---|
| Structure | Anything with a definite size and shape that holds up a load or holds something in. |
| Solid / frame / shell | Solid material all the way through, like a dam / a skeleton of joined parts, like a bicycle / a thin outer layer around a space, like an egg. |
| Load | A force a structure must hold up. Dead load: its own weight. Live load: everything it carries. Static: still or slowly changing. Dynamic: moving or changing fast. |
| Force / newton (N) | A push or a pull, measured in newtons. A small apple weighs about 1 N. |
| Mass / weight | The amount of matter, in kilograms / the force of gravity on it, in newtons — on Earth, about 9.8 N for each kilogram. |
| External / internal force | Acting on a structure from outside, like a load / acting inside its materials as they carry the load. |
| Compression / tension | Squeezing a material together / stretching it apart. |
| Shear / torsion | Pushing two parts in opposite directions so one slides past the other / twisting. |
| Joint | Where parts meet. Fixed joints — nails, screws, glue, welds — hold them still; moving joints, such as hinges, let them move. |
| Strength / flexibility / durability | How big a load a material takes before breaking / how far it bends without breaking / how long it lasts. |
| Centre of gravity / stability | The point where all of an object's weight seems to act / how well a structure resists tipping or collapsing. |
| Triangulation | Bracing a frame with triangles, which cannot change shape unless a side breaks or stretches. |
| Specifications / margin of safety | What a design must do: its load, size, materials, cost / extra strength built in beyond the largest load expected. |
Kinds of structures and what they are made of
The idea: Solid structures are strong because of their mass, frames because of how their parts are joined, and shells because of their curved outer layer. Most real structures combine two or three.
| Type | How it holds a load | Examples |
|---|---|---|
| Solid | Its own mass of material | A concrete dam, a brick wall, a sandcastle |
| Frame | Parts joined into a skeleton | A house's wooden frame, a transmission tower, a spider web, your skeleton |
| Shell | A thin, usually curved layer that spreads the load over its surface | An egg, a snail's shell, a pop can, a hockey helmet |
Combinations. A tent is a frame of poles holding up a thin fabric shell. A house is a wooden frame on a solid concrete foundation.
Choosing materials. Engineers match the material to the force. Concrete is very strong when squeezed but cracks easily when stretched, so concrete beams have steel bars, called rebar, set inside to take the stretching. Steel is strong both ways. Wood is much stronger along its grain than across it. Rubber is flexible and glass is not.
Joints. Fixed joints hold parts still: nails, screws, bolts, glue, welds. Moving joints let them move on purpose: a door's hinges, a bicycle chain, your knees and elbows. A joint is often a structure's weakest point, which is why a wobbly chair is usually loose where a leg meets the seat.
Forces on a structure and in it
The idea: External forces act on a structure from outside. Inside, they become compression, tension, shear and torsion — and every failure is one of those forces winning.
weight (N) = mass (kg) × 9.8 · roughly 10 N for every kilogram
Worked: mass and weight. A student has a mass of 50 kg. On Earth her weight is 50 × 9.8 = 490 N. On the Moon, where gravity is about one-sixth as strong, she would weigh only about 80 N — but her mass would still be 50 kg. There is just as much of her; gravity pulls on it less.
Sorting loads. A bridge's own steel and concrete are a dead, static load. A truck driving across is a live, dynamic load. Snow sitting on a roof is live but static.
Spotting them. The legs of a table are in compression. The cables of a suspension bridge are in tension, like a rope in tug-of-war. A hole punch cuts paper by shear. Wringing out a cloth and turning a key are torsion.
Bending is two forces at once. Lay a plank across two bricks and stand on the middle: its top face is squeezed shorter and its bottom face stretched longer. On a diving board, fixed at one end only, it is the other way round — the top stretches and the bottom squeezes.
How structures fail.
- Buckling — compression makes a long, thin part fold sideways. Stand on an empty pop can and it crumples.
- Bending — a beam sags until it cracks, usually starting on the stretched face.
- Shearing — one part slides off another, as when a bolt snaps straight across.
- Twisting — torsion wrings a part out of shape until it gives way.
Stability, strength and good design
The idea: A structure stays up if its centre of gravity stays over its base, and stays strong if its shape sends the forces where its materials can take them. Designers check both by testing against the specifications.
Stability. A structure tips when its centre of gravity moves outside its base. A low centre of gravity and a wide base make it stable — which is why you crouch and spread your feet on a moving bus.
Shapes that add strength without much weight.
- Triangles. Push on the corner of a square frame and it folds into a slanted shape; a triangle cannot. Look at the side of Edmonton's High Level Bridge, a steel frame finished in 1913, or any transmission tower: triangles everywhere.
- Arches turn a load pressing down into compression along the curve, pushing out and down into the ground at each end. Stone and concrete, strong in compression, make good arches.
- I-beams put most of their material at the top and bottom, where bending squeezes and stretches hardest.
- Corrugation — folding a thin sheet into ridges — makes cardboard and metal roofing far stiffer than a flat sheet.
- Reinforcing adds a stronger material where it is needed, like rebar in concrete.
Worked: comparing two bridges. The specifications: span a 40 cm gap using only paper and glue. Bridge A has a mass of 30 g and holds 45 N before it fails. Bridge B has a mass of 80 g and holds 60 N. B held more, but compare the load each gram of paper carried:
A: 45 N ÷ 30 g = 1.5 N per gram · B: 60 N ÷ 80 g = 0.75 N per gram
Each gram of A carries twice as much. If the specifications reward strength for the mass, A wins; if they ask only for the strongest bridge, B does.
Margin of safety. A deck expected to carry at most 4000 N of people and furniture might be built to hold 12 000 N — three times as much — because loads can be bigger than planned, materials vary, and wood and steel weaken over the years. The more lives depend on a structure, the larger its margin of safety should be.
What costs marks
The idea: Six, and the first costs marks in every grade from here on.
- Giving weight in kilograms. Weight is a force, in newtons; mass is in kilograms.
- Swapping the two faces of a bent beam. Supported at both ends: top squeezed, bottom stretched.
- Calling every failure “breaking”. Name it: buckling, bending, shearing or twisting.
- Judging a design only by the load it held. Check what the specifications reward.
- Treating a square frame as rigid. Only triangles hold their shape.
- Forgetting the dead load. A structure has to hold itself up before it holds anything else.