Forces in structures

Compression, tension and torsion, struts and ties, types of structures and loads. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

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Forces in practice

The three main forces in Grade 7: compression, tension and torsion (and shear).

Situation	Force
A diving board bends when someone stands on the end	Tension on top, compression underneath
The ropes of a tent, pegged into the ground	Tension
A washing line sags	Tension: pull the line tighter

A wobbly square becomes rigid with a diagonal strut.

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Types of structures and stability

Type	Description	Example
Frame	Parts joined, like a skeleton	Pylon, roof truss
Shell	Hollow with a thin outer layer	Egg, helmet
Solid	Heavy block that resists by its mass	Dam wall
Frame + shell	A frame with a shell over it	A car

Tall structures have a wide base. A bookcase stays up with heavy books low down and fixed to the wall.
Fold or roll paper into tubes to hold a book; the folds in corrugated iron make thin metal stiff.
A water tank on a stand: legs braced with cross-pieces into triangles. Bridges have deep foundations in firm ground.
Strong but light bicycle frame: aluminium tubes.

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Remember this and test yourself

Remember this: your summary

Compression squashes; tension stretches; torsion is a twisting force.
A strut resists compression; a tie resists tension.
Triangles keep their shape: add a diagonal strut to a wobbly square.
Static load does not move (own weight); dynamic load moves (cars over a bridge).
Biomimicry: engineers copy nature, like a honeycomb for light, strong panels.

Write these points in your book. Read them again the night before the test.

Memory trick

Strut = Squash. Tie = Tension (pull).

Flash cards · Tap to turn over

Important words

First say what the word means, then turn the card over.

Test yourself · Part 2

True or false?

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