What forces act on products, and how can materials be reinforced to withstand them?
How materials and objects are affected by forces and stresses including tension, compression, bending, torsion and shear, and how materials can be reinforced and stiffened to resist them.
A focused answer to AQA GCSE Design and Technology specialist principle on forces and stresses, covering tension, compression, bending, torsion and shear, and the techniques used to reinforce and stiffen materials.
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What this dot point is asking
This is AQA section 3.2.2. AQA wants you to name the forces and stresses that act on materials and objects, and explain how products are reinforced or stiffened to resist them. You need to identify which force is acting in a given situation and describe a suitable strengthening technique. In Paper 1 this is examined by giving you a loaded object and asking you to name the force and a matching reinforcement method.
The five forces and stresses
A material can be strong in one type of force but weak in another. Concrete is strong in compression but weak in tension, which is why steel rods (strong in tension) are added to make reinforced concrete for beams. The first step in any forces question is to picture what is pulling, pushing, twisting or bending the object: a tow rope is in tension, a table leg in compression, a shelf in bending, a screwdriver shaft in torsion, and a bolt holding two plates that are pulled apart is in shear.
Reinforcing and stiffening
The key idea examiners reward is that you can resist a force far more efficiently by changing shape than by simply adding bulk, because a deeper or folded section resists bending much better for the same mass. Techniques include:
- Lamination: bonding thin layers together, as in plywood or laminated beams, so the layers resist bending and the grain directions cancel out weaknesses.
- Webbing and ribs: adding internal supports (ribs on a plastic moulding, a web in an I-beam) that spread load and resist bending without filling the whole volume.
- Folding and corrugation: bending sheet material into folds, channels or corrugations greatly increases stiffness; corrugated card and roofing sheet resist bending far better than a flat sheet of the same material.
- Triangulation: building triangles into a frame, because a triangle is rigid (it cannot change shape without changing a side length) whereas a rectangle can rack into a parallelogram. Adding a diagonal turns rectangles into triangles.
- Adding fibres or another material: glass or carbon fibres in a resin (GRP, carbon fibre) resist tension; steel reinforcing bars give concrete tensile strength.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA flat shelf made from acrylic sags in the middle when loaded with books. Identify the force causing this and describe two ways the shelf could be reinforced or stiffened to resist it.Show worked answer →
A 4-mark question gives a mark for the force and marks for two valid, explained techniques.
The force is bending: the weight of the books pushes the middle of the shelf down while the supported ends resist, so the shelf curves (the top surface is in compression, the bottom in tension).
Way 1, add a lip or fold along the front edge (or bend the acrylic into an L or U section). Changing the cross-section greatly increases stiffness without adding much material, because the depth of the section resists bending.
Way 2, add a rib or webbing underneath, or laminate a second layer onto the shelf. Increasing the effective thickness or bonding layers together resists the sag.
Markers reward (1) naming bending, (2) and (3) two named techniques (folding, ribs, webbing, lamination, a deeper section) each with why it helps. Saying only "make it thicker" earns one mark at most.
AQA 20183 marksExplain why triangulation is used to strengthen a frame structure such as a bridge or a shelf bracket.Show worked answer →
A 3-mark Explain wants the geometry and its effect.
A rectangle can distort into a parallelogram under a sideways load because its corners can pivot, so a rectangular frame is unstable. A triangle cannot change shape without changing the length of one of its sides, so it is rigid. Adding a diagonal member to a frame splits each rectangle into two triangles, which locks the frame and lets it carry load without collapsing or racking.
In a shelf bracket the diagonal carries the load in compression or tension straight back to the wall, supporting the shelf. Markers reward (1) a triangle is rigid or cannot deform, (2) a rectangle can distort, (3) the diagonal turns rectangles into triangles to carry the load. Saying only "triangles are strong" caps the mark.
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Sources & how we know this
- AQA GCSE Design and Technology (8552) specification — AQA (2017)