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.

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

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.

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.

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:

A shelf loaded with books bends — the top of the shelf is compressed and the bottom is stretched.
A diving board bends when a diver stands on the end.
A cantilever chair (like a Breuer Cesca chair) relies on the metal tube frame resisting bending.

Beam Condition	Top Surface	Bottom Surface
Load applied from above	Compression	Tension
Load applied from below	Tension	Compression

AQA Exam Tip: A very common exam question asks you to identify where tension and compression occur in a bending beam. Remember: the side nearest the load is in compression, and the opposite side is in tension. Draw a simple diagram with labelled arrows to support your answer.

Torsion

Torsion is a twisting force. It occurs when one end of a material is rotated relative to the other end, or when rotational forces act in opposite directions.

Examples of torsion:

Wringing out a wet cloth applies torsion — you twist each end in opposite directions.
A screwdriver shaft experiences torsion when turning a screw.
The crankshaft of a car engine is under torsion from the rotation of the pistons.
A wind turbine shaft transfers rotational energy through torsion.

Materials that must resist torsion need good torsional strength and shear strength (since torsion creates shear stresses within the material). Solid circular cross-sections (like a steel rod) resist torsion well because the material is distributed evenly around the axis of rotation.

Shear

Shear is a force that causes one part of a material to slide past an adjacent part. Shear forces act in parallel but opposite directions, like the blades of a pair of scissors.

Examples of shear:

Scissors cutting paper — the two blades apply shear forces to the paper.
A rivet or bolt joining two plates experiences shear if the plates are pulled in opposite directions.
Guillotine cutting of sheet metal applies shear force along the cutting line.
A beam resting on two supports with a load in the middle experiences shear forces at the supports.

Materials with high shear strength resist being cut or torn. Ductile metals like mild steel have good shear strength; brittle materials like cast iron and ceramics have poor shear strength and may fail suddenly under shear.

Forces in Real Products

Most real products experience combinations of these forces simultaneously. A designer must identify which forces are most significant and select materials accordingly.

Product	Primary Forces	Suitable Material	Why
Bicycle frame	Tension, compression, bending	Aluminium alloy, chromoly steel, carbon fibre	High strength-to-weight ratio, resists combined forces
Scissors	Shear	High-carbon steel	Excellent shear strength, holds a sharp edge
Shelving bracket	Bending, tension	Steel	High tensile and compressive strength
Suspension bridge cable	Tension	Steel wire rope	Extremely high tensile strength
Building column	Compression	Reinforced concrete	Excellent compressive strength
Spanner handle	Torsion, bending	Chrome vanadium steel	Resists twisting and bending without deforming

Forces Acting on Materials: Tension, Compression, Bending, Torsion, Shear