Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define and differentiate between types of forces on structures.
• Identify and explain the function of different structural members.
• Describe different types of mechanisms and calculate mechanical advantage.
• Choose appropriate materials based on their properties.
• Practice sketching simple mechanisms and structures.

Lesson summary

This week, we're going to consolidate our understanding of key Grade 8 Technology concepts, specifically focusing on topics relevant to engineering design, structures, and mechanisms, ensuring you are well-prepared for upcoming assessments and real-world problem-solving. Understanding these concepts is crucial because they form the foundation for further studies in fields like engineering, architecture, and even trades like plumbing and carpentry. In South Africa, a strong foundation in technology equips you to contribute meaningfully to infrastructure development, sustainable solutions, and technological advancements that improve our lives.

Lesson notes

2.1 Forces on Structures Structures are subjected to various forces that can cause them to deform or even fail. Understanding these forces is crucial for designing safe and stable structures.

Tension: A pulling force that tends to stretch a material. Think of a rope being pulled tight in a tug-of-war or a cable supporting a bridge.

Compression: A pushing force that tends to squash or compress a material. Examples include the weight of a building pushing down on its foundation or a pillar supporting a roof.

Shear: A force that acts parallel to a surface, causing the material to slide or deform. Imagine cutting paper with scissors – the blades exert a shear force.

Torsion: A twisting force that causes rotation. Think of twisting a screwdriver to tighten a screw or wringing out a wet cloth.

Bending: A combination of tension and compression forces that cause a material to curve. The top surface experiences tension (stretching), while the bottom surface experiences compression (squashing). A bridge with a car driving over it experiences bending.

Example: Consider a simple wooden bridge. The weight of people crossing the bridge creates a bending force. The top of the bridge experiences tension, while the bottom experiences compression. The supports (pillars) of the bridge experience compression. 2.2 Structural Members Different structural members play specific roles in supporting and distributing loads.

Strut: A structural member designed to resist compression. Think of the legs of a table or the vertical supports in a frame.

Tie: A structural member designed to resist tension. Cables in suspension bridges or ropes holding up a tent are good examples.

Beam: A horizontal structural member designed to resist bending. Bridges, floors, and roofs all use beams.

Column: A vertical structural member designed primarily to resist compression. Pillars in buildings are columns.

Example: In a house, the walls act as columns (supporting the roof), while the wooden beams supporting the roof and floor resist bending forces. The cables in a suspended ceiling act as ties, resisting tension. 2.3 Mechanisms Mechanisms are devices used to transfer motion and force. They allow us to perform tasks more easily or efficiently.

Levers: Simple machines that use a pivot point (fulcrum) to multiply force. There are three classes of levers, depending on the relative positions of the fulcrum, load, and effort.

Mechanical Advantage (MA) of a lever: MA = Load / Effort = Distance from Effort to Fulcrum / Distance from Load to Fulcrum

Example: Using a crowbar to lift a heavy rock. The crowbar acts as a lever, with the fulcrum being the point where the crowbar touches the ground. By applying a small effort, we can lift a much heavier load.

Linkages: Systems of rigid bars connected by joints that transmit motion. They can change the direction, magnitude, or type of motion.

Example: A bicycle brake system. When you squeeze the brake lever, the linkage transmits the motion to the brake pads, which then apply friction to the wheel.

Gears: Toothed wheels that mesh together to transmit rotary motion. They can change the speed, torque, and direction of rotation.

Gear Ratio: Number of teeth on Driven Gear / Number of teeth on Driving Gear If the driven gear has more teeth than the driving gear, the speed decreases, but the torque increases. If the driven gear has fewer teeth than the driving gear, the speed increases, but the torque decreases.

Example: A bicycle uses gears to change the speed and effort required to pedal. Using a lower gear (larger rear gear) makes it easier to climb a hill but reduces the speed.

Pulleys: Wheels with a grooved rim that are used with a rope or cable to lift objects or transmit force.

Mechanical Advantage (MA) of a pulley: Equal to the number of rope segments supporting the load.

Example: Using a pulley system to lift a bucket of water from a well. The more ropes supporting the bucket, the less force you need to apply to lift it.

Example: Gear Calculations Gear A has 20 teeth and is connected to Gear B, which has 40 teeth. Gear A rotates at 100 RPM (Revolutions Per Minute). What is the gear ratio? Gear Ratio = Number of teeth on Driven Gear / Number of teeth on Driving Gear = 40 / 20 = 2 What is the speed of Gear B? Since the gear ratio is 2, Gear B rotates slower. Speed of Gear B = Speed of Gear A / Gear Ratio = 100 RPM / 2 = 50 RPM 2.4 Materials Choosing the right material for a technological application is crucial. Different materials have different properties that make them suitable for specific purposes.

Strength: Ability to withstand force without breaking.

Flexibility: Ability to bend without breaking.

Durability: Ability to withstand wear and tear over time.

Cost: The price of the material.

Example: For building a bridge, we need a material with high strength (like steel or reinforced concrete) and durability.