Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define and differentiate structures, mechanisms, and systems with South African examples.
• Identify the 5 types of forces acting on structures.
• Describe the functions of different mechanisms and calculate mechanical advantage.
• Explain how system components (input, process, output) work together.
• Apply this knowledge to evaluate simple technological solutions.

Lesson summary

This week in Technology, we will be focusing on revising and consolidating concepts we covered in the previous weeks of Grade

7. Technology plays a crucial role in shaping our everyday lives in South Africa, from the cellphones we use to connect with family and friends, to the infrastructure that provides us with electricity and clean water. Understanding basic technological principles allows us to be informed citizens who can contribute to solving local challenges and participating in a growing digital economy. We will specifically revisit structures, mechanisms, and systems, linking them together to see how they function in practical applications.

Lesson notes

2.1 Structures: A structure is something that supports a load or encloses a space.

There are three main types of structures: Frame Structures: These structures are made up of individual members joined together to form a rigid framework. Examples include bridges, towers, and buildings with a skeleton frame. In South Africa, the Nelson Mandela Bridge in Johannesburg is an excellent example of a frame structure. The steel beams are joined together to form a strong framework capable of supporting the weight of traffic.

Shell Structures: These structures are hollow and derive their strength from their shape. Examples include eggs, domes, and aeroplane fuselages. In South Africa, a traditional Zulu hut ("ikhukwane") can be considered a basic shell structure; although they often involve frame elements too. Modern shell structures, like certain stadium roofs, are advanced examples.

Solid Structures: These are structures made from a solid block of material. Examples include dams, monuments, and some walls. The Gariep Dam (formerly Hendrik Verwoerd Dam) is a massive solid structure built to hold back water and generate hydroelectric power.

Forces Acting on Structures: Structures are subjected to different types of forces that can affect their stability. It's vital to consider them in designs.

Tension: A pulling force that stretches or elongates a material. Think of a rope being pulled tight or the cables supporting a suspension bridge.

Compression: A pushing force that shortens or compresses a material. The legs of a table experience compression from the weight of objects on the table.

Shear: A force that causes one part of a material to slide past another. Consider scissors cutting paper – that's shear force in action.

Torsion: A twisting force. Tightening a bolt with a wrench applies torsion.

Bending: A combination of tension and compression that causes a material to curve. A diving board bends when someone stands on it; the top surface experiences tension, while the bottom experiences compression. 2.2 Mechanisms: A mechanism is a device that changes the direction or magnitude of a force or motion.

Levers: A simple machine that uses a rigid bar pivoted around a fixed point (fulcrum) to multiply force. There are three classes of levers, depending on the relative positions of the fulcrum, load, and effort. Think of using a crowbar to lift a heavy rock – that's a lever in action. Someone sweeping a floor is using a lever (the broom).

Linkages: A system of rigid bars connected by joints, used to transmit motion and force. A bicycle's brake mechanism involves linkages to transfer the force from the brake lever to the brake pads.

Gears: Toothed wheels that mesh together to transmit rotary motion and change speed or torque. Gears are used in cars, bicycles, and clocks.

Wheels and Axles: A simple machine consisting of a wheel attached to a smaller axle. Turning the wheel requires less force than turning the axle directly. Think of turning a steering wheel in a car – it's much easier than turning the steering shaft directly.

Pulleys: A wheel with a grooved rim around which a rope or cable passes. Pulleys are used to lift heavy objects and change the direction of force. Construction workers use pulleys to lift materials to upper floors of buildings. Using a well to draw water utilises a pulley.

Mechanical Advantage (MA): This is the ratio of the output force (load) to the input force (effort).

Levers: MA = Length of effort arm / Length of load arm Pulleys: For a simple pulley system, MA is approximately equal to the number of rope segments supporting the load.

Example: Consider a lever where the effort arm is 2 meters long and the load arm is 0.5 meters long. The mechanical advantage is 2 / 0.5 =

4. This means that a force of 10N applied to the effort arm will lift a load of 40N. 2.3 Systems: A system is a set of interacting components that work together to achieve a specific goal. Systems have inputs, processes, and outputs.

Input: What goes into the system (e.g., energy, materials, information).

Process: What happens to the input within the system (e.g., a series of mechanical operations, chemical reactions, or information processing).

Output: What comes out of the system (e.g., a product, service, or result).

Example: A simple water purification system.

Input: Dirty water.

Process: Filtration and disinfection to remove impurities and bacteria.

Output: Clean, drinkable water. Another

Example: Consider a bicycle.

Input: Human energy (pedaling).

Process: The pedals turn the gears, which turn the wheels, propelling the bicycle forward.

Output: Movement of the bicycle. Guided Practice (With Solutions)

Question 1: Identify the type of structure (frame, shell, or solid) used in a traditional rondavel (round hut) in South Africa. Explain your reasoning.

Solution: A traditional rondavel is primarily a shell structure.