Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define 'structure' and identify local examples.
• Describe four basic forces: tension, compression, shear, and torsion.
• Explain how forces affect a structure's strength.
• Understand the concepts of load and support.
• Investigate how different shapes impact strength.

Lesson summary

Welcome to Grade 7 Technology! This term, we will be exploring the fascinating world of structures. Structures are all around us, from the bridges we cross to the houses we live in and even the chairs we sit on. Understanding how structures work is important because it allows us to design and build things that are safe, strong, and useful. Here in South Africa, strong and reliable structures are crucial for infrastructure development, housing, and ensuring safety in our communities. Imagine designing a shack that can withstand strong winds in the Cape or building a bridge that connects rural communities to essential services.

Lesson notes

What is a Structure? A structure is something that has a definite size and shape, that is used for a particular purpose and that has the ability to support a load. It's anything that can support a force, whether it's a building, a bridge, a table, or even a simple beam. Think about a traditional South African rondavel; it's a structure designed to provide shelter. Or a simple shebeen, built to serve as a gathering place. All of these are structures that serve a purpose.

Forces Acting on Structures: Forces are pushes or pulls that can change the shape or movement of an object. Here are the four main types of forces that act on structures: Tension: Tension is a pulling force that stretches a material. Think of a rope being pulled taut or the cables on a suspension bridge.

A practical example: the wires supporting electricity poles experience tension due to the weight of the wires and the force of the wind.

Compression: Compression is a pushing force that squeezes or shortens a material. Think of the legs of a table supporting the weight of the tabletop and anything placed on it or the pillars holding up a building. Consider the bricks in the foundation of a house experiencing compressive forces from the weight of the house above.

Shear: Shear is a force that causes layers of a material to slide past each other. Imagine cutting paper with scissors; the blades apply a shear force.

A real-world example: when an earthquake shakes the ground, the foundations of buildings can experience shear forces. The rivets connecting steel plates in a bridge also experience shear forces.

Torsion: Torsion is a twisting force. Think of twisting a wet cloth to wring out the water or twisting the lid off a jar. The drive shaft in a car experiences torsion as it transfers power from the engine to the wheels. A simple example is twisting a piece of paper.

Load and Support: Load: A load is any force that acts on a structure. It can be the weight of objects placed on the structure (dead load), the weight of the structure itself (self-weight), or external forces like wind or people (live load). The load will cause internal stresses inside the structure.

Examples: The weight of people crossing a bridge is a load. The weight of corrugated iron sheets on a shack is a load.

Support: A support is anything that resists the forces acting on a structure, keeping it stable. Supports transfer the load of a structure into the ground. The foundations of a building are supports. The pillars of a bridge are supports.

Shapes and Strength: The shape of a structure significantly affects its strength. Some shapes are naturally stronger than others.

For example: Triangles: Triangles are very strong because their rigid shape distributes forces evenly. This is why you often see triangular supports in bridges and roof trusses.

Arches: Arches distribute weight outwards, making them very strong for supporting heavy loads. Consider the arches used in historical buildings and bridges.

Cylinders: Cylinders are strong under compression. Think of concrete pillars supporting a building.

Worked example

Scenario: A group of learners builds a small bridge out of cardboard for a class project. They place books on the bridge to test its strength. What type of force are the books applying to the bridge?

Answer: The books are applying a compressive force to the bridge. The weight of the books is pushing down on the bridge, compressing the cardboard.

Scenario: Consider a suspension bridge. The cables are anchored to the ground and run over towers, supporting the road deck below. What type of force is primarily acting on these cables?

Answer: The cables are primarily experiencing tension. The weight of the road deck and any traffic on it pulls on the cables, stretching them.

Scenario: A builder is constructing a wall using bricks. What type of force primarily exists between the bricks?

Answer: The bricks are primarily experiencing compression. The weight of the bricks above is pushing down on the bricks below, squeezing them together. The mortar acts as a key to help distribute the load more evenly, reducing shear.

Guided Practice (With Solutions)

Question 1: Identify the main type of force acting on the following:

a) The legs of a chair when someone sits on it.

b) A rope used to tow a car.

c) The axle of a bicycle wheel when pedaling.

Solution: