Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define & classify structures and their properties.
• Explain simple mechanisms & calculate mechanical advantage.
• Describe the components of a control system.
• Design a simple structure to meet specific criteria.
• Construct a simple mechanical system.

Lesson summary

This week in Technology, we will be revising and consolidating key concepts covered in the previous weeks. Specifically, we will be focusing on structures, mechanisms, and control systems. Understanding these topics is vital because they form the foundation of how things around us work. From the simple gate latch on a farm to complex machinery in a factory, these principles are constantly in play. In a developing country like South Africa, having a solid understanding of these concepts empowers you to innovate, solve problems, and contribute to the growth of our industries and infrastructure.

Lesson notes

Structures A structure is anything that has a definite size and shape and is used to support a load. Structures can be natural (like a mountain) or man-made (like a bridge).

Types of Structures: Solid Structures: Made entirely of solid material (e.g., a brick, a statue, a concrete pillar). They resist forces because the material itself is strong.

Frame Structures: Consist of separate parts joined together to form a framework (e.g., a bicycle frame, a building scaffolding, the support structure of a cellphone tower). They are strong because of the way the parts are arranged.

Shell Structures: Use a thin, carefully shaped outer layer to provide strength and rigidity (e.g., an eggshell, a car body, a water tank). They are strong because of their shape.

Properties of Structures: Strength: The ability of a structure to withstand forces without breaking or collapsing.

Stability: The ability of a structure to remain upright and not topple over. A stable structure has a low center of gravity and a wide base.

Stiffness: The ability of a structure to resist bending or deforming under load.

Example 1: Imagine a shack in a township. Is it a solid, frame, or shell structure? It's typically a frame structure, built with wooden planks or metal sheets forming a supporting framework.

Example 2: A traditional rondavel. Is it a solid, frame, or shell structure? The walls are essentially solid structures, supporting the roof's weight. The roof itself could be seen as a type of shell structure. Mechanisms A mechanism is a device that changes motion or force. Mechanisms are used to make work easier by changing the amount of force required, the distance over which the force is applied, or the direction of the force.

Simple Mechanisms: Levers: A rigid bar that pivots around a fixed point (fulcrum). Examples include seesaws, crowbars, and bottle openers. There are three classes of levers depending on the position of the fulcrum, load, and effort.

Gears: Toothed wheels that mesh together to transmit motion and force. Gears can increase or decrease speed and change the direction of rotation.

Pulleys: Wheels with a grooved rim around which a rope or cable runs. Pulleys can change the direction of a force and can also reduce the amount of force required to lift a load (depending on the number of ropes supporting the load).

Mechanical Advantage (MA): The ratio of the output force (load) to the input force (effort). A mechanical advantage greater than 1 means that the mechanism multiplies the force.

Lever MA: MA = Distance from fulcrum to effort / Distance from fulcrum to load Gears MA (Gear Ratio): Gear Ratio = Number of teeth on driven gear / Number of teeth on driving gear Example 1: A builder uses a wheelbarrow (lever). If the load is 0.5m from the wheel (fulcrum) and the builder applies effort 1.5m from the wheel, the mechanical advantage is 1.5/0.5 =

3. This means they need to apply one-third of the force to lift the load compared to lifting it directly.

Example 2: A bicycle has gears. If the front gear (driving gear) has 48 teeth and the back gear (driven gear) has 16 teeth, the gear ratio is 16/48 = 1/3 or 0.

3

3. This means the back wheel rotates three times faster than the pedals. Control Systems A control system is a system that manages, commands, directs, or regulates the behavior of other devices or systems.

Basic Components: Input: A signal or information that is fed into the system (e.g., pressing a button, a change in temperature).

Process: The action or operation that the system performs based on the input (e.g., the electrical circuit activating the motor, a thermostat comparing the current temperature to the desired temperature).

Output: The result of the process (e.g., the motor starts, the heater turns on).

Types of Control Systems: Open-Loop Control System: The output has no effect on the input. (e.g., a toaster: you set the timer (input), and the toaster heats up for that amount of time regardless of how brown the toast is).

Closed-Loop Control System: The output affects the input. (e.g., a thermostat in a house: the thermostat measures the temperature (input), compares it to the set temperature (process), and turns the heater on or off (output). The temperature of the room (output) then affects the thermostat's measurement (input), creating a feedback loop).

Example 1: A traffic light. The timer is the input, the sequence of lights changing (red, yellow, green) is the process, and the lights themselves are the output. This is an example of an open-loop control system because the traffic flow does not change the timer settings.

Example 2: A geyser with a thermostat. The thermostat setting (desired temperature) is the input. The geyser heating the water is the process. The hot water is the output. The water temperature affects the thermostat (feedback), which turns the heating element on or off, maintaining the desired temperature. This is an example of a closed-loop control system.