Lesson Notes By Weeks and Term v5 - Grade 7
Structures: forces and strength in structures – Week 3 focus
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Structures: forces and strength in structures – Week 3 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Technology
Class: Grade 7
Term: 3rd Term
Week: 3
Theme: General lesson support
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This week, we delve into the fascinating world of structures and the forces that act upon them. Understanding how forces affect structures is crucial because it helps us design and build things that are strong, stable, and safe. From houses and bridges to cell phone towers and school buildings, everything around us is a structure designed to withstand certain forces. In South Africa, where we face challenges like extreme weather conditions in some areas (heavy rains, strong winds) and the need for durable infrastructure, understanding structural integrity is especially important.
What is a Structure? A structure is anything that is built or constructed from parts put together in a planned way. It is designed to support a load or resist forces. Buildings, bridges, tables, chairs, and even the human skeleton are all examples of structures.
Forces Acting on Structures: Tension: A pulling force that stretches or elongates a material. Think of pulling a rope in a tug-of-war. The rope is under tension.
Compression: A pushing force that squeezes or shortens a material. Imagine a stack of books pressing down on the book at the bottom. The bottom book is under compression.
Shear: A force that causes one part of a material to slide past another part. Think of cutting paper with scissors. The blades apply shear force.
Torsion: A twisting force. Imagine twisting a wet towel to wring out the water. The towel is experiencing torsion.
Bending: A combination of tension and compression. When a beam is bent, the top surface experiences compression, while the bottom surface experiences tension.
Example: Imagine a simple wooden bridge.
When a car drives across it: The weight of the car causes compression on the top of the bridge. The weight of the car causes tension on the bottom of the bridge. If the bridge is not properly supported, it could bend and eventually break. Wind blowing against the side of the bridge can apply shear forces to the supporting pillars.
Structural Strength and Stability: Strength: The ability of a structure to withstand forces without breaking or deforming permanently. A stronger material can withstand greater forces.
Stability: The ability of a structure to maintain its shape and position without collapsing or toppling over. A stable structure resists overturning or buckling.
Factors Affecting Strength and Stability: Material Properties: Different materials have different strengths and weaknesses. Steel is strong in both tension and compression, while concrete is strong in compression but weak in tension. Wood's strength varies depending on the grain and type of wood. In South Africa, different materials are used depending on availability and cost. For example, brick and concrete are common building materials, while wood is often used for furniture and roofing.
Shape: The shape of a structure significantly affects its strength and stability. Triangles are known for their rigidity and are often used in bridge and roof designs. Arches are excellent at distributing compressive forces.
Load Distribution: How the weight (load) is distributed across a structure affects its stability. A load concentrated in one spot can cause a structure to fail more easily than a load spread evenly across its surface.
Example: A rectangular piece of cardboard can easily bend under a load.
However, if you fold the cardboard into a triangular prism, it becomes much stronger and more resistant to bending. This demonstrates how shape can significantly impact structural strength.