The big idea: Compression squashes, tension stretches, shear slides one part across another, torsion twists, and bending does two of them at once.
Name a force by what the material actually DOES, not by where the load came from.
Each force drawn as the deformation it causes — then the step showing that bending is tension and compression together.
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| Force | What the material does | What resists it best |
|---|---|---|
| Compression | Squashed shorter by two forces pushing inwards on the same line | Concrete, stone, brick — but a slender member buckles long before it crushes |
| Tension | Stretched longer by two forces pulling outwards on the same line | Steel, rope, fibres — concrete and stone are very weak and must be reinforced |
| Shear | Slides apart along a plane, pushed in opposite directions across itself | A bolt, rivet, pin or glue line — and it is what scissors do |
| Torsion | Twisted about its long axis, one end turned relative to the other | A CLOSED tube; slot that tube along its length and it becomes dramatically weaker |
| Bending | One face stretched, the opposite face squashed, with a neutral axis between | Section DEPTH far more than material: doubling the depth is worth roughly eight times |
Bending is not a fifth kind of force: A loaded beam is in tension along one face and compression along the other, with a neutral axis between them where the stress is zero.
That is why an I-beam has thick flanges and a thin web — the middle was carrying almost nothing.
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Static forces
- Steady and unchanging
- The structure's own weight
- A shelf of books, a parked car, a fixed tank of water
- Sized against yield strength, with a safety factor
Dynamic forces
- Changing in size, direction or position
- A person walking across a floor
- Wind gusting, a door slammed, a wheel over a kerb
- Sized against fatigue — which is a much lower limit
Why dynamic loads are so much worse: A part cycled a million times can fail at a small fraction of the load it would carry steadily for ever. That is fatigue, and it starts at a scratch, a sharp corner or a tool mark.
So anything that moves is designed with generous radii, no sharp internal corners and a smooth surface — and it is inspected, because fatigue cracks grow slowly before they break.
A dropped load is not the same as a placed one: Lowering a 20 kg load onto a shelf applies about 20 kg.
Dropping the same load from a height applies several times that for an instant, because the structure has to absorb its kinetic energy as well as hold it. That is why a step is rated for a person stepping, not jumping.
How this is tested — how forces act within a structure, and static against dynamic. It comes up two ways:
Paper 1 — multiple choice
- Identify the force acting on a described member.
- Classify a stated load as static or dynamic.
Paper 2 — analysing a product
- Explain the forces acting in a named product under load.
- Explain why a dynamic load governs the design of a part.
The trap: Calling everything compression or tension. A bolt through two plates being pulled apart is in SHEAR, and a drive shaft is in TORSION — name the force by what the material does.
Explain the forces acting in a bicycle when a rider pedals hard out of the saddle, and identify which are dynamic.
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