Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define and differentiate between types of forces with SA examples.
• Identify and explain frame, shell, and solid structures.
• Describe the function of beams, columns, struts, and ties.
• Analyze simple mechanical systems like levers.
• Explain the importance of materials selection in structural design.

Lesson summary

This week focuses on consolidating our understanding of key Technology concepts covered throughout the term, specifically those relating to structures and mechanisms. Think about how vital technology is in South Africa – from building houses to designing transport systems, understanding structures and mechanisms is crucial for building a better future. We will be reviewing topics such as forces, types of structures (natural and man-made), structural members, and mechanical systems. A firm grasp of these concepts will not only help you ace your exams, but also enable you to contribute meaningfully to technological advancements in our country.

Lesson notes

Forces Forces are pushes or pulls that can change the shape, speed, or direction of an object. Understanding forces is fundamental to understanding how structures behave.

Tension: A pulling force, like pulling a rope in a tug-of-war. In a South African context, think of the cables holding up the Bloukrans Bridge (the highest commercial bridge bungee jump in the world). Those cables are under tension.

Compression: A squeezing force, like a stack of bricks. The columns supporting the roof of a shack are under compression. If the load is too great the column can buckle.

Shear: A force that causes parts of an object to slide past each other, like cutting paper with scissors. Think of the forces acting on the bolts connecting two steel plates in a building during an earthquake; they experience shear.

Torsion: A twisting force, like twisting a towel to wring out water. The axles of a car experience torsion as the engine's power turns the wheels.

Bending: A force that causes a structure to curve. A diving board bends when someone stands on the end. A beam supporting a roof bends under the weight of the roof.

Types of Structures Frame Structures: These structures are made up of individual members connected together to form a rigid framework.

Examples: Buildings, bridges, radio towers. In South Africa, many modern buildings are frame structures made of steel and concrete.

Shell Structures: These structures are thin, curved surfaces that distribute forces throughout the entire structure.

Examples: Eggshells, domes, aeroplane fuselages. Think of the dome of the Union Buildings in Pretoria; it’s a shell structure.

Solid Structures: These structures are made from a single, solid piece of material.

Examples: Dams, statues, solid concrete walls. The walls of a traditional rondavel house can be considered solid structures.

Structural Members Beams: Horizontal structural members that resist bending. They support loads applied along their length.

Examples: The horizontal steel beams supporting a bridge deck, wooden beams supporting a roof.

Columns: Vertical structural members that resist compression. They support vertical loads from above.

Examples: Pillars supporting a building, the legs of a table.

Struts: Slender members used to resist compression, often found in trusses. Think of the diagonal members in a roof truss; they are struts.

Ties: Slender members used to resist tension. The cables in a suspension bridge are ties.

Mechanical Systems: Levers A lever is a simple machine that uses a rigid object (the lever arm) and a pivot point (the fulcrum) to multiply force. There are three classes of levers, defined by the relative positions of the fulcrum, load, and effort.

Class 1 Lever: Fulcrum is between the load and the effort.

Example: A seesaw.

Class 2 Lever: Load is between the fulcrum and the effort.

Example: A wheelbarrow.

Class 3 Lever: Effort is between the fulcrum and the load.

Example: Tweezers.

Mechanical Advantage (MA): The ratio of the output force (load) to the input force (effort). MA = Load/Effort. A mechanical advantage greater than 1 means the lever multiplies your force.