Forces Acting on Materials

This lesson covers the five main types of force that act on materials in products and structures, as specified in AQA GCSE D&T (8552), Section 3.2.2. Understanding these forces is essential for selecting appropriate materials and designing products that are safe and fit for purpose. Every product you use — from a chair to a bridge to a pair of scissors — is subjected to forces during use, and the designer must ensure the chosen materials can withstand those forces.

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The Five Types of Force

There are five main types of force that act on materials. Each force applies stress to the material in a different way.

Force	Description	Direction of Force	Symbol (Simplified)
Tension	Pulling or stretching force	Forces pull away from each other	<-- -->
Compression	Squashing or crushing force	Forces push towards each other	--> <--
Bending	Combination of tension and compression	Force applied perpendicular to the length	Beam bows downward
Torsion	Twisting force	Rotational forces in opposite directions	Twist arrows
Shear	Sliding or cutting force	Parallel forces in opposite directions	Scissors motion

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Tension

Tension is a force that stretches or pulls a material apart. When a material is under tension, its molecules are being pulled away from each other.

Examples of tension in everyday products:

• A washing line supporting wet clothes is under tension — the weight of the clothes pulls the line downward while the posts hold the ends up.
• The cables of a suspension bridge are under tension — they support the deck by being stretched between the towers and anchors.
• A guitar string is under tension — it is stretched between the tuning pegs and the bridge.
• An elastic band when stretched is experiencing tension.

Materials suitable for resisting tension must have high tensile strength. Steel cables, Kevlar fibres, and nylon rope all have excellent tensile strength.

AQA Exam Tip: In diagrams, tension forces are shown as arrows pointing AWAY from the material (pulling outward). If you are asked to draw a force diagram, always show the direction of the force with clear arrows.

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Compression

Compression is a force that squashes or pushes a material together. When a material is under compression, its molecules are being pushed closer together.

Examples of compression in everyday products:

• The legs of a table are under compression — the weight of the tabletop and objects on it pushes down on the legs.
• The foundations of a building are under compression from the weight of the structure above.
• A car tyre is under compression where it contacts the road surface.
• The piers (vertical supports) of a bridge are under compression from the weight of the deck and traffic.

Materials suitable for resisting compression must have high compressive strength. Concrete, brick, stone, and cast iron all perform well under compression.

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Bending

Bending occurs when a force is applied perpendicular (at right angles) to the length of a material, causing it to curve or deflect. Bending is actually a combination of tension on one side and compression on the other side of the material.

When a beam bends:

• The top surface is under compression (being squashed shorter).
• The bottom surface is under tension (being stretched longer).
• The neutral axis (middle) experiences neither tension nor compression.

Examples of bending: