Lesson Notes By Weeks and Term v5 - Grade 7
Structures: forces and strength in structures – Week 3 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
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
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
This week, we delve into the heart of structures: forces and the strength required to withstand them. Structures are all around us, from the houses we live in to the bridges we cross and the towers that hold cellphone signals. Understanding how forces affect structures and how to build strong structures is crucial for ensuring safety and stability in our daily lives. Imagine a shack settlement constantly battling wind damage or a poorly built bridge collapsing – these are real consequences of not understanding the principles we'll be exploring.
Forces in Structures Forces are pushes or pulls that can cause an object to move, change shape, or change direction. In structures, forces are always present. Understanding these forces is critical to designing safe and stable structures.
Tension: This is a pulling force that stretches a material. Imagine pulling a rope – the rope is under tension. Cables in suspension bridges (like those sometimes seen in the Drakensberg mountain range) are primarily under tension.
Example: A rope holding up a swing. The rope is experiencing a pulling force due to the weight of the person on the swing.
Compression: This is a squeezing force that compresses a material. Imagine stacking bricks – the bricks at the bottom are under compression.
Example: The pillars supporting a roof. The pillars are experiencing a squeezing force from the weight of the roof.
Shear: This 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: A bolt connecting two pieces of wood. If the wood pieces are pulled in opposite directions, the bolt experiences a shear force trying to cut it.
Torsion: This is a twisting force. Imagine wringing out a wet cloth – the cloth is experiencing torsion.
Example: Turning a screwdriver. The screwdriver shaft experiences torsion.
Bending: This is a combination of tension and compression. Imagine placing a heavy book in the middle of a long, thin ruler supported at its ends – the ruler bends. The top of the ruler is under tension (being stretched), and the bottom of the ruler is under compression (being squeezed).
Example: A beam supporting a floor. The top of the beam experiences compression, and the bottom experiences tension. Structural Shapes and Strength The shape of a structure plays a significant role in its strength and ability to distribute forces.
Triangles: Triangles are incredibly strong because they are rigid. The fixed angles prevent them from deforming easily under stress. This is why triangular shapes are often used in trusses, bridges, and roof structures. Think of the steel structures of Eskom power lines; they are almost always based on triangular designs.
Example: Bridges often use triangular trusses for support. The triangular shape distributes the weight of the bridge and the traffic across it, making it stronger.
Arches: Arches distribute compressive forces outwards along the curve of the arch. This allows them to support heavy loads over large spans. Roman arches are famous examples, but you can also see them in some older buildings in cities like Cape Town.
Example: A brick archway over a doorway. The arch distributes the weight of the bricks above it outwards to the supporting walls.
Squares/Rectangles: While not as inherently strong as triangles, squares and rectangles can be made stronger through bracing (adding diagonal supports). These shapes are commonly used in buildings and furniture.
Example: The frame of a table. While the frame is typically square or rectangular, it needs to be sturdy to support the weight placed on it.
Cylinders: Cylinders are strong under compression and can resist bending forces. Think of the cardboard tube inside a roll of paper towels.
Example: Steel pipes used in construction are strong under compression. Materials and Strength The material a structure is made from also affects its strength. Different materials have different properties: Strength: How much force a material can withstand before it breaks.
Stiffness: How much a material will deform under a given force.
Density: How much mass is packed into a given volume.
Common materials include: Wood: Relatively lightweight and easy to work with, but can rot or burn. Used extensively in housing, especially in rural areas.
Steel: Very strong and durable, but heavy and can rust. Used in bridges, high-rise buildings, and factories. Consider the Moses Mabhida Stadium in Durban; it uses a lot of steel.
Concrete: Strong under compression and relatively inexpensive, but can crack under tension. Used in foundations, walls, and roads.
Brick: Strong under compression and fire-resistant, but heavy and brittle. Commonly used for building walls.
Plastic: Lightweight and resistant to corrosion, but can be weak and melt at high temperatures. Used in piping and some structural components. Methods for Strengthening Structures Bracing: Adding diagonal supports to prevent deformation, especially in rectangular structures. Think of adding diagonal wooden supports to a fence.
Reinforcing: Adding stronger materials to weaker ones, such as steel rebar in concrete. This significantly increases the tensile strength of concrete.
Thickening: Increasing the thickness of structural members to increase their resistance to forces. A thicker beam can support more weight than a thinner one.
Laminating: Bonding layers of materials together to create a stronger composite material, such as plywood.