Structures: forces, stability, strength, frameworks, reinforcement

Forces

Forces are pushes or pulls that can change the motion of an object. Think of a tug‑of‑war: each team pulls with a force, and the rope feels the tension.

• Weight (gravity) – pulls objects toward Earth. $W = mg$ where $m$ is mass and $g$ is 9.81 m/s².
• Normal force – the support force from a surface, e.g., a book resting on a table.
• Friction – resists motion between surfaces. Rougher surfaces mean more friction.
• Applied force – any external push or pull, like a hand lifting a box.

⚙️ Analogy: Imagine a seesaw. The weight of a child on one side creates a downward force; the seesaw’s support provides an upward normal force. The balance depends on the forces and their distances from the pivot.

Stability

Stability is the ability of a structure to resist tipping or collapsing. It depends on:

1. Centre of gravity – the point where all mass is considered to be concentrated.
2. Base of support – the area beneath the structure that contacts the ground.
3. Load distribution – how forces spread across the structure.

🏗️ Example: A tall tower is stable if its centre of gravity is low and its base is wide. If you lean a tower too far, the centre of gravity moves outside the base, causing it to topple.

Strength

Strength is a material’s ability to withstand loads without breaking. Key concepts:

• Compressive strength – resistance to crushing.
• Tensile strength – resistance to pulling apart.
• Shear strength – resistance to sliding forces.

📐 Analogy: Think of a rubber band. If you pull too hard, it stretches until it snaps. The maximum stretch before snapping is its tensile strength.

Frameworks

Frameworks are the skeletons that give structures shape and support. Common types:

🛠️ Tip: Triangles are the strongest shape because they maintain their form even when forces are applied.

Reinforcement

Reinforcement adds extra strength to structures, especially where stresses are high.

1. Steel bars (rebar) – used in concrete to resist tensile forces.
2. Fibres (e.g., carbon, glass) – lightweight yet strong, used in composites.
3. Bracing – diagonal members that prevent sway.

💪 Example: A reinforced concrete bridge uses steel bars inside the concrete to prevent cracking when heavy trucks pass over.

Exam Tips

• Understand the difference between force, stress, and strain – they’re often mixed up.
• When analysing a structure, always identify the load path – where forces travel through the members.
• Use diagrams: sketch the structure, label forces, and show the centre of gravity.
• Remember the strength‑to‑weight ratio – lighter materials can still be strong if designed correctly.
• Practice converting units (kg to N, m to cm) – many questions require you to work in SI units.