Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify four main forces: tension, compression, shear, and torsion.
• Explain how shapes like triangles and arches add strength.
• Describe how different materials withstand forces.
• Understand how and why structures can fail.
• Learn how to prevent structural failure.

Lesson summary

This week, we delve into the fascinating world of structures and the forces that act upon them. Understanding how forces affect structures is crucial, not only for building safe and reliable structures like houses and bridges, but also for understanding how everyday objects, from tables to cell phone towers, stay upright. Imagine a shack built without considering the forces of wind or the weight of the roof - it's likely to collapse! Similarly, a poorly designed bridge across the Orange River could be a disaster.

Lesson notes

Forces and Structures: A structure is anything that can support a load. This could be a massive bridge, a simple chair, or even the bones in your body. Forces are pushes or pulls that act on objects. When forces act on structures, they can cause them to change shape (deform) or even break.

Types of Forces: There are four main types of forces that act on structures: Tension: Tension is a pulling force that stretches or elongates a material. Think of a rope being pulled in a tug-of-war. The rope is under tension. Cables on suspension bridges are also under tension. A simple example would be trying to stretch an elastic band.

Compression: Compression is a pushing force that squeezes or shortens a material. Imagine stacking books on top of each other. The bottom books are under compression. The columns supporting the roof of a building are under compression. Think of trying to squash a marshmallow.

Shear: Shear is a force that acts parallel to a surface, causing one part of the material to slide past another part. Imagine cutting paper with scissors. The scissors apply a shear force to the paper. The force between tectonic plates in the Earth's crust is a shear force.

Torsion: Torsion is a twisting force. Imagine twisting a wet cloth to wring out the water. The cloth is under torsion. The force applied when tightening a nut with a wrench is torsion.

Structural Shapes and Strength: The shape of a structure plays a crucial role in its strength and stability. Some shapes are better at withstanding certain types of forces than others.

Triangles: Triangles are incredibly strong and rigid shapes. This is because the angles in a triangle cannot change without changing the length of the sides. Triangles are used in many structures, such as bridges, roof trusses, and towers, to provide stability. Think of the support structure for electricity pylons – they are often triangular.

Arches: Arches are curved structures that distribute weight evenly downwards and outwards. They are particularly strong under compression. Arches are used in bridges, doorways, and other structures. The Drakensberg mountains have many natural arch formations.

Columns: Columns are vertical supports designed to withstand compressive forces. They are commonly used in buildings and other structures. The strength of a column depends on its material, shape, and height.

Materials and Strength: The material used to build a structure significantly affects its ability to withstand forces. Different materials have different strengths and weaknesses.

Steel: Steel is a strong and durable material that is commonly used in bridges, buildings, and other large structures. It is good at withstanding both tension and compression.

Concrete: Concrete is a strong material that is good at withstanding compression. It is often reinforced with steel to increase its tensile strength. Concrete is used in foundations, walls, and floors.

Wood: Wood is a relatively lightweight and strong material that is commonly used in houses and other smaller structures.

Brick: Brick is a strong material for compression but has less tensile strength.

Structural Failure: Structures can fail when the forces acting on them exceed their strength. There are several ways in which a structure can fail: Buckling: Buckling occurs when a column or beam collapses under compression. This often happens when a column is too tall or too thin.

Shearing: Shearing occurs when a material breaks due to a shear force.

Fracture: Fracture occurs when a material breaks due to tension, compression, or shear.

Preventing Structural Failure: There are several ways to prevent structural failure: Use strong materials: Choose materials that are appropriate for the forces that the structure will be subjected to.

Use strong shapes: Incorporate strong shapes, such as triangles and arches, into the design.

Provide adequate support: Ensure that the structure is adequately supported.

Consider the environment: Take into account the environmental conditions, such as wind, rain, and earthquakes, that could affect the structure. Regular maintenance is vital to identify problems early.