Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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.

Performance objectives

• Define and identify forces like tension, compression, and shear.
• Describe how shapes and materials affect a structure's strength.
• Explain how loads affect structures.
• Design and test a simple structure.
• Analyze forces on common South African structures.

Lesson summary

This week, we delve into the fascinating world of structures and how forces act upon them. Understanding structures is essential because they are all around us, from the houses we live in to the bridges we cross every day. In South Africa, the development of strong and reliable structures is crucial for infrastructure development, housing, and even creating safe recreational spaces. Learning about forces and strength will help us appreciate the engineering behind these structures and understand why some are stronger and more durable than others. We'll investigate how different forces, like pushing and pulling, affect a structure’s stability and explore ways to enhance its strength.

Lesson notes

What is a Structure? A structure is anything that has a definite size and shape, and whose main purpose is to support a load or resist a force. Buildings, bridges, furniture, and even the human skeleton are all examples of structures.

Forces: The Pushes and Pulls A force is a push or a pull that can cause an object to change its speed, direction, or shape. Several types of forces are crucial to understanding how structures behave: Tension: This is a pulling force. Imagine pulling on a rope; the rope is under tension. In a structure, tension can occur in cables supporting a bridge or the ropes holding up a tent.

Compression: This is a squeezing or pushing force. Think about standing on the ground; the ground is compressing under your weight. In a structure, columns and walls often experience compression.

Shear: This force occurs when one part of a material is pushed or pulled in one direction while another part is pushed or pulled in the opposite direction. Imagine cutting paper with scissors; the scissors are applying shear force. Earthquakes can exert shear forces on buildings.

Torsion: This is a twisting force. Think of twisting a bottle cap. Screws holding wooden planks together in a fence will experience torsion.

Strength in Structures: A structure's strength refers to its ability to withstand forces without breaking or deforming excessively. Several factors influence the strength of a structure: Material: Different materials have different strengths. Steel is generally stronger than wood, and wood is stronger than paper. The choice of material is crucial for the intended load of a structure. Consider zinc vs. clay brick for a house build in a rural area.

Shape: The shape of a structure can significantly affect its strength. Certain shapes, like triangles and arches, are naturally stronger because they distribute forces more evenly.

Design: The way a structure is designed and built also plays a crucial role in its strength. A well-designed structure will distribute forces evenly, minimizing stress on any one point.

Load: The load refers to the weight or force that a structure is designed to support. Structures must be designed to withstand the expected load safely.

Worked example

Example 1: Tension in a Clothesline

Imagine a clothesline strung between two poles. When you hang wet clothes on the line, the weight of the clothes creates a pulling force (tension) in the line. The greater the weight of the clothes, the greater the tension in the line.

Why: The clothes are pulled down by gravity and the line opposes that force to keep the clothes from falling.

How to make it stronger: Use a thicker clothesline (stronger material) or reduce the distance between the poles (less length under tension).

Example 2: Compression in a Brick Wall

Think about a brick wall. The weight of the bricks above each layer presses down on the bricks below. This creates a compression force within the wall.

Why: The weight of the bricks above each layer of brick applies a vertical force to the lower layers of bricks.

How to make it stronger: Use stronger bricks or widen the base of the wall to better distribute the load.