Structures and Forces
FoundationStructures carry five forces: compression, tension, bending, torsion and shear. Arches, frames, beams and boxes each carry loads differently, and reshaping members turns weaknesses into strengths.
- Structures carry five forces: compression, tension, bending, torsion and shear
- Common types — arch, frame, beam and box — each carry loads differently
- A wide base and low centre of gravity keep structures balanced
文體小知識：科技知識說明文有條理地講解結構、力和負載等概念，配合實例說明抽象原理，使讀者能夠理解及應用。
§Text Passage
1Bridges, buildings, tables and chairs, bird cages — even a sheet of paper — are structures: any shape that holds its form and carries forces. The job of a structure is to carry its load, the weight and forces placed upon it, and to pass them safely down to the ground or its supports. Hong Kong's Tsing Ma Bridge spans a busy channel and the IFC towers into the sky precisely because their structures are well designed.
2Five forces commonly act on structures. Compression squeezes a member inward; tension stretches it outward; bending makes it curve; torsion twists it like a towel being wrung; shear slides two parts in opposite directions, like scissors. Different parts of one structure can feel different forces at the same time. Sit on a wooden bench: the seat bends, the legs are compressed, and the joints feel shear when the bench wobbles.
3Common structure types are arches, frames, beams and box structures. An arch passes weight along its curve to both ends and is very strong; a frame is a network of members with triangles, light yet rigid; a beam spans between supports and resists bending directly; a box structure is closed on all sides and resists twisting especially well. The tower of a great bridge is a frame, while a double-decker bus body is a box — and each carries loads differently.
4Changing the shape and arrangement of members can hugely strengthen a structure without changing the material. Fold a sheet of paper into pleats or roll it into a tube and it will hold up books, because the folds convert bending into compression and tension between creases. The triangle is the only polygon that keeps its shape under load, which is why triangles appear everywhere in pylons and bamboo scaffolding. Only by analysing forces can we design structures that are both light and stable.
1Summary 重點摘要
Structures carry five forces: compression, tension, bending, torsion and shear. Arches, frames, beams and boxes each carry loads differently, and reshaping members turns weaknesses into strengths.
2Vocabulary cards Vocabulary
An arrangement of parts that carries and passes on forces.
The weight and forces placed on a structure.
A force that squeezes a member inward.
A force that stretches a member outward.
A force that makes a member curve.
A twisting force on a member, like wringing a towel.
A force that slides two parts of a member in opposite directions, like scissors.
A structure that passes weight along a curve to both ends.
A skeleton structure of members, usually with triangles.
A member spanning between supports that resists bending.
3Step by step Steps
- Structures and loadsA structure is an arrangement that holds its shape and carries forces, while a load is the weight or force placed on it. Every structure must pass its load safely to the supports so that products and buildings do not collapse. For example, when the upper deck of a double-decker bus is full, the load travels through the body to the wheels and then to the road; when you carry a schoolbag, its straps and back panel form the structure bearing the load. Always ask first: where does the force come from, and where must it go?
- Compression and tensionCompression squeezes a member so it becomes shorter and thicker; tension stretches it so it becomes longer and thinner. They act in opposite directions yet often appear together. For example, the towers of a suspension bridge are pressed down by the cables — compression — while the cables themselves are stretched by the deck — tension. Sitting on a wooden bench, the seat bends under your weight: its top surface is compressed and its underside is stretched. Knowing the direction of forces is the first step in choosing member shapes and materials.
- Bending, torsion and shearBending makes a long member curve in the middle and is the commonest cause of failure in thin plates and long beams. Torsion twists a member, its two ends turned in opposite directions like a towel being wrung. Shear makes two parts of a member slide in opposite directions, as scissors do to paper. For example, a shelf full of books bends downwards, a towel twists when wrung dry, and paper shears when cut. Long thin members fear bending, so they are thickened, folded or supported; screws at the joints must resist shear.
- Four common structure typesBy shape and the way they carry forces, structures can be grouped into arches, frames, beams and box structures. An arch passes weight along its curve to the supports at both ends, common in bridges and tunnels. A frame is built of members, usually with triangles, light yet rigid. A beam spans between columns and resists bending directly. A box structure is closed on all sides and resists twisting and impact. For example, an arch footbridge, a pylon or scaffold frame, the beam over a classroom door and a refrigerated container each suit a different need and carry loads differently.
- Balance and stabilityTo stand firm, a structure must keep all forces balanced: the downward load equals the upward support, and the combined force passes through the base. The wider the base and the lower the centre of gravity, the more stable the structure. For example, a roly-poly toy has a round heavy bottom and a very low centre of gravity, so it always rights itself; roller-coaster supports are wide and low so the ride stays balanced at speed. Shelves, lamp posts and even fan bases all use the same wide-base, low-centre principle to stay safe and durable.
- Shaping for strengthWithout changing the material, changing the shape of members greatly increases load capacity. Fold a flat sheet into pleats, roll it into a tube or add triangular creases, and bending is converted into compression and tension between the creases. The triangle is the only polygon that keeps its shape under load, so adding triangles adds stiffness. For example, the wavy middle layer of corrugated board makes it stiff, a foil tray becomes firm once its edges are folded, and folding gates use diagonal members forming triangles — all trading shape for strength.
- Input–process–output in mechanismsIn mechanical design, the black-box method analyses a device as input–process–output: input and output are often force and motion, and motion takes forms such as straight-line and rotation; inside, the mechanism transforms the direction or size of the input force to produce the required action. For example, in a bicycle brake, squeezing the lever is the input, the cable and levers magnify the force as the process, and the wheel being clamped to a stop is the output. Ignoring the inside and comparing the two ends prepares you for the next two lessons on systems and simple machines.
4Diagrams Diagrams
5Worked example Example
Example: laid flat, a sheet of A4 paper cannot even hold a pen. How can it be changed to hold a textbook?
- Analyse the failure → a flat sheet is thin; the book's weight bends it at once
- Fold into waves (accordion) → the creases turn the sheet into slanting struts, forming triangles
- Forces transformed → bending is replaced by compression and tension between creases
- Improve further → more and deeper folds mean higher capacity; or roll it into a tube
- Answer: Folding the A4 sheet lengthways into accordion waves, or rolling it into a tube, lets it hold a textbook. A flat sheet bends as a whole and collapses; folded into waves, each crease acts like a small strut, converting bending into compression and tension between creases, and the forces travel along the folds to the supports. More and deeper folds carry more. Strength comes from shape as much as from material.
6Common mistakes Mistakes
7Quick check Quick check
Q1A towel hangs on a hook. What force does the hook feel? A. compression B. tension C. torsion
Q2Why do pylons and scaffolding use triangular frames everywhere?
Q3What force acts on card when you cut it with scissors?
Q4A long shelf sags in the middle. Which force causes it, and how can it be improved?
Q5Which type of structure is a double-decker bus body? What is its advantage?
8Recap Recap
- Structures carry five forces: compression, tension, bending, torsion and shear
- Common types — arch, frame, beam and box — each carry loads differently
- A wide base and low centre of gravity keep structures balanced
- The triangle keeps its shape under load — the key to rigidity
- Changing shape (folding, rolling, adding edges) greatly raises load capacity