Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify and classify frame and shell structures.
• Explain food processing techniques like preservation.
• Describe mechanisms like levers, gears, and pulleys.
• Analyse the impact of technology on society.
• Apply your understanding to solve design problems.

Lesson summary

This week, we're consolidating all the key technology concepts you've learned in Grade

7. This is super important because technology is EVERYWHERE! From the cell phones you use to chat with your friends, to the internet that connects you to the world, to the cars that transport your families, technology shapes our lives every single day. Understanding technology helps you become a critical thinker, a problem-solver, and potentially, an innovator who can contribute to South Africa's future. Think about the challenges facing our communities – water scarcity, power shortages, unemployment.

Lesson notes

2.1 Structures: Frames and Shells Structures are designed to withstand forces and support loads.

There are two main types: Frames: These are structures made up of individual components (beams, columns, struts, ties) joined together to create a rigid framework. Think of a bridge (like the Nelson Mandela Bridge in Johannesburg), a building (like the Ponte City Apartments), or even a bicycle frame. Frame structures distribute weight across the framework to provide strength and stability. Key concepts related to frame structures are tension (a pulling force), compression (a pushing force), and shear (a sliding force). The shape and arrangement of the components significantly affect the strength of the frame. Triangles are incredibly strong shapes in frame structures because they distribute forces evenly.

Shells: These are structures made up of a continuous outer surface that provides strength and stability. Think of an eggshell, a car body, or the roof of the Moses Mabhida Stadium in Durban. Shell structures distribute forces across the entire surface, making them very strong for their weight. The shape of the shell is crucial. Curved shapes, like domes and arches, are particularly strong because they distribute forces evenly. A weak point in a shell can cause the entire structure to collapse.

Example 1 (Frame): Consider a simple wooden gate. The vertical and horizontal pieces form a frame. The vertical pieces are under compression (supporting the weight of the gate), and the horizontal piece is under tension (preventing the gate from sagging). A diagonal brace, forming a triangle, would significantly strengthen the gate.

Example 2 (Shell): Think of a corrugated iron sheet used for roofing in many South African homes. The corrugated shape (series of curves) makes it much stronger than a flat sheet of iron because it acts as a shell structure, distributing the force of the wind and rain more effectively. 2.2 Mechanisms: Levers, Gears, and Pulleys Mechanisms are devices that transmit or modify motion and force. Understanding how they work is vital for designing tools and machines.

Levers: A lever is a simple machine that consists of a rigid bar that pivots around a fixed point called a fulcrum. Levers are used to multiply force. There are three classes of levers depending on the relative positions of the fulcrum, load, and effort.

Class 1: Fulcrum in the middle (e.g., seesaw, pliers).

Example in SA: Using a crowbar to lift a heavy rock.

Class 2: Load in the middle (e.g., wheelbarrow, nutcracker).

Example in SA: A wheelbarrow being used at a construction site.

Class 3: Effort in the middle (e.g., tweezers, fishing rod).

Example in SA: Using a fishing rod while fishing in a dam.

Mechanical Advantage (MA) of a Lever: MA = Load / Effort. A MA > 1 means the lever multiplies force.

Gears: Gears are toothed wheels that mesh together to transmit rotary motion. They can be used to change speed, torque (rotational force), and direction of rotation. When two gears of different sizes mesh, the smaller gear rotates faster than the larger gear, but the larger gear exerts more torque.

Gear Ratio: The ratio of the number of teeth on the driven gear (output) to the number of teeth on the driving gear (input). Gear Ratio = Number of teeth on driven gear / Number of teeth on driving gear.

Pulleys: A pulley is a wheel with a grooved rim around which a rope, belt, or cable passes. Pulleys are used to lift heavy loads and can change the direction of a force.

Fixed Pulley: Changes the direction of the force but does not multiply the force (MA = 1).

Example in SA: A clothes line pulley system.

Movable Pulley: Multiplies the force but does not change the direction. Mechanical Advantage (MA) of a Pulley System: Approximately equal to the number of rope sections supporting the load.

Example 3 (Gears): A bicycle uses gears to transfer the power from the pedals to the rear wheel. By changing gears, a cyclist can adjust the speed and effort required to pedal. 2.3 Food Processing Food processing involves transforming raw ingredients into consumable food products. This can involve various techniques aimed at preserving food, improving its taste, texture, or nutritional value.

Preservation: Methods used to prevent spoilage and extend the shelf life of food.

Common methods include: Drying: Removing moisture to inhibit microbial growth (e.g., biltong, dried fruit).

Canning: Sealing food in airtight containers and heating it to kill microorganisms.

Freezing: Slowing down microbial growth and enzymatic activity.

Salting: Using salt to draw out moisture and inhibit microbial growth (e.g., droëwors).

Smoking: Using smoke to add flavour and preserve food.

Pasteurization: Heating liquids (e.g., milk) to kill harmful bacteria.

Modification: Changing the characteristics of food through various processes.

Fermentation: Using microorganisms to convert carbohydrates into acids or alcohol (e.g., amasi, bread).