Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify forces like tension, compression, shear, torsion, and bending.
• Explain how materials and shapes affect strength.
• Design a simple, strong structure.
• Analyse structures in your community.

Lesson summary

This week, we delve into the fascinating world of structures and how forces act upon them. Understanding forces and the strength of different structures is crucial for understanding the world around us. From the houses we live in to the bridges we cross, structures are designed to withstand various forces. Knowing about these forces helps us to appreciate the engineering that goes into these structures and also allows us to design and build our own strong and stable constructions. In South Africa, with its diverse landscapes and growing infrastructure, understanding structures is essential for sustainable development and safe construction practices.

Lesson notes

What is a Structure? A structure is any arrangement of materials designed to support a load. This could be anything from a simple table to a massive building. Structures provide stability and resistance to external forces.

Types of Forces: Understanding the different types of forces is fundamental to understanding how structures behave.

Tension: Tension is a pulling force that stretches or elongates a material. Imagine pulling on a rope – the rope is experiencing tension.

Example: The cables supporting the Bloukrans Bridge, the highest commercial bungee jumping bridge in the world, are under immense tension.

Compression: Compression is a squeezing force that shortens or compresses a material. Imagine pushing down on a table leg – the leg is experiencing compression.

Example: The pillars supporting a building are under compression from the weight of the floors above.

Shear: Shear is a force that causes one part of a material to slide past another part. Imagine cutting paper with scissors – the paper is experiencing shear force.

Example: The rivets holding together steel beams in a bridge experience shear forces.

Torsion: Torsion is a twisting force that causes a material to rotate. Imagine twisting a bottle cap – the bottle cap is experiencing torsion.

Example: The axle of a car experiences torsion as it transfers power from the engine to the wheels.

Bending: Bending is a combination of tension and compression. When a beam bends, the top surface is under compression, and the bottom surface is under tension.

Example: A bridge deck when a car drives over it experiences bending.

Material Strength: Different materials have different abilities to resist these forces. This is known as their strength.

Tensile Strength: How well a material resists tension. Steel has high tensile strength. Think of steel cables used in bridges.

Compressive Strength: How well a material resists compression. Concrete has high compressive strength. Think of the concrete pillars in buildings.

Shear Strength: How well a material resists shear. Wood can have good shear strength along the grain. Think of wooden beams used in roof structures.