Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define and classify structures and identify forces acting on them.
• Explain common mechanisms like levers, gears, and pulleys.
• Describe the components of a simple control system.
• Identify South African examples of these technologies.
• Apply these principles to design and evaluate simple solutions.

Lesson summary

This week, we're consolidating key concepts from Grade 8 Technology that are essential for building a strong foundation for future learning and for understanding the technological world around us, especially in the South African context. A solid grasp of these concepts will empower you to critically analyze everyday technology, design simple solutions to practical problems, and contribute to technological innovation in our country. Specifically, we will be revisiting structures, mechanisms and control systems.

Lesson notes

2.1 Structures A structure is something that has a definite size and shape, and performs a function. Structures can be natural (like a tree or a mountain) or man-made (like a bridge or a building).

There are three main types of structures: Frame Structures: These are made of separate parts joined together to form a rigid framework. Think of a soccer goalpost, a bicycle frame, or the steel skeleton of a building. They are strong because the force is distributed across the frame. Many South African shack structures utilize frame structures.

Shell Structures: These are hollow and curved, providing strength and stability due to their shape. Examples include eggshells, domes, and aircraft fuselages. The roof of the Nelson Mandela Bay Stadium in Port Elizabeth is a shell structure.

Solid Structures: These are made from a solid piece of material. They are often strong but can be heavy. Examples include a brick, a statue, or a dam wall. The walls of many traditional South African houses are solid structures made of mud or brick.

Forces acting on Structures: Tension: A pulling force that stretches a material.

Compression: A pushing force that squeezes a material.

Shear: A force that causes parts of a material to slide past each other.

Torsion: A twisting force.

Bending: A force that causes a material to curve. Understanding these forces helps engineers design structures that can withstand the loads they will experience. For example, the supports of a bridge experience compression, while the cables experience tension. 2.2 Mechanisms A mechanism is a device that changes motion or force. Common mechanisms include levers, gears, and pulleys. They allow us to perform tasks more easily.

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

First-class lever: Fulcrum between the load and effort (e.g., a seesaw, a pair of scissors).

Second-class lever: Load between the fulcrum and effort (e.g., a wheelbarrow, a bottle opener).

Third-class lever: Effort between the fulcrum and load (e.g., tweezers, a fishing rod).

Mechanical Advantage (MA) of a Lever: MA = Load / Effort = Distance from Effort to Fulcrum / Distance from Load to Fulcrum Gears: Gears are toothed wheels that mesh together to transmit rotary motion. They can change the speed, torque, and direction of rotation.

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 If the gear ratio is greater than 1, the speed decreases, and the torque increases. If the gear ratio is less than 1, the speed increases, and the torque decreases.

Pulleys: Pulleys are wheels with a grooved rim around which a rope or cable passes. They can change the direction of a force and can also provide mechanical advantage. Mechanical Advantage (MA) of a Pulley System: In theory, the MA of a pulley system is equal to the number of rope sections supporting the load.

However, friction reduces the actual MA. Pulleys are used extensively in South African mines. 2.3 Control Systems A control system is a system that controls another system.

It consists of three main components: Input: The information or signal that is fed into the system (e.g., a button being pressed, a sensor reading a temperature).

Process: The action or operation that the system performs based on the input (e.g., a computer program, a circuit).

Output: The result or effect of the process (e.g., a light turning on, a motor starting).

Example: A simple electric kettle.

Input: Turning on the switch.

Process: The heating element heats the water.

Output: The water boils and the kettle switches off. Control systems can be open-loop (no feedback) or closed-loop (feedback). A thermostat is a closed-loop system because it monitors the temperature and adjusts the heating accordingly. Guided Practice (With Solutions)

Question 1: Identify the type of structure (frame, shell, or solid) and the main force acting on the following: A cellphone tower.

Solution: The cellphone tower is a frame structure. The main force acting on it is bending due to wind. The frame design allows it to withstand this force by distributing it across the entire structure.

Question 2: A gear system has a driving gear with 20 teeth and a driven gear with 60 teeth. Calculate the gear ratio. If the driving gear rotates at 120 rpm, what is the speed of the driven gear?

Solution: Gear Ratio = Number of teeth on driven gear / Number of teeth on driving gear = 60 / 20 = 3 This means the driven gear rotates 3 times slower than the driving gear. Speed of driven gear = Speed of driving gear / Gear Ratio = 120 rpm / 3 = 40 rpm.

Question 3: Design a simple control system for an automatic gate at a school entrance.

Solution: Input: A sensor detects a car approaching the gate.