Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Explain advanced mechanical systems like gear trains and levers.
• Analyse electrical circuits with resistors in series and parallel.
• Explain simple automated systems combining mechanical and electrical parts.
• Differentiate between open-loop and closed-loop control systems.
• Design a simple system using a sensor, controller, and actuator.

Lesson summary

This week, we delve deeper into mechanical and electrical systems, building on your existing knowledge of basic circuits and mechanisms. We'll be focusing on how these systems work together to create more complex and useful devices. Understanding these principles is crucial for future innovators and problem-solvers. Imagine being able to design more efficient water pumps for rural communities or creating smart traffic management systems to alleviate congestion in our cities – this knowledge is the foundation. In a country like South Africa, with diverse technological challenges, understanding how to build and control systems is an essential skill.

Lesson notes

2. 1.

Advanced Mechanical Systems: Gear Trains, Levers, and Linkages Mechanical systems are all about transferring forces and motion. We've looked at simple machines before, but now we're putting them together.

Gear Trains: Gears are toothed wheels that mesh together to transmit rotary motion. A gear train is a series of gears working together. They can change the speed, torque (turning force), and direction of rotation.

Gear Ratio: The gear ratio is 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` A gear ratio greater than 1 means the output speed is slower, but the output torque is higher (speed reduction, torque amplification). This is useful for lifting heavy objects. A gear ratio less than 1 means the output speed is faster, but the output torque is lower (speed increase, torque reduction). This is useful for increasing the speed of a rotating fan.

Example: A gear train has a driving gear with 20 teeth and a driven gear with 40 teeth. The gear ratio is 40/20 =

2. This means the driven gear rotates twice as slow as the driving gear, but with twice the torque.

Levers: Levers are rigid bars that pivot around a fixed point called a fulcrum. They're used to multiply force. There are three classes of levers, depending on the relative positions of the fulcrum, load (resistance), and effort (force).

Mechanical Advantage (MA) of a Lever: MA = Load / Effort. An MA greater than 1 means the lever makes it easier to lift the load.

Example: Imagine using a crowbar (a type of lever) to lift a heavy rock. The fulcrum is the point where the crowbar rests on the ground. If you apply a small force (effort) at the end of the crowbar, you can lift a much heavier rock (load).

Linkages: Linkages are systems of rigid bars connected by joints. They're used to transmit motion and change its direction or type.

Example: Bicycle brakes use a linkage system. When you squeeze the brake levers, the linkage pulls on the brake cable, which in turn activates the brakes on the wheels. Another example is a windscreen wiper mechanism in a car. 2.

2. Advanced Electrical Systems: Resistors in Series and Parallel, Ohm's Law Resistors in Series: When resistors are connected in series, the current flows through each resistor one after the other.

Total Resistance (R T ): R T = R 1 + R 2 + R 3 + ...

Example: A circuit has two resistors in series: R 1 = 10 ohms and R 2 = 20 ohms. The total resistance is R T = 10 + 20 = 30 ohms.

Resistors in Parallel: When resistors are connected in parallel, the current has multiple paths to flow through.

Total Resistance (R T ): 1/R T = 1/R 1 + 1/R 2 + 1/R 3 + ...

Example: A circuit has two resistors in parallel: R 1 = 10 ohms and R 2 = 20 ohms. 1/R T = 1/10 + 1/20 = 3/

2

0. Therefore, R T = 20/3 = 6.67 ohms (approximately).

Ohm's Law: This fundamental law relates voltage (V), current (I), and resistance (R).

Formula: V = I R (Voltage = Current * Resistance)

Example: If a circuit has a voltage of 12V and a resistance of 6 ohms, the current is I = V/R = 12/6 = 2 Amps. 2.

3. Simple Automated Systems and Control Systems Automated System: A system that operates without direct human control, usually through a combination of mechanical and electrical components.

Example: An automatic irrigation system for a farm. A timer turns on a pump (electrical component) which drives a water distribution system (mechanical component).

Control System: A system that regulates another system.

There are two main types: Open-Loop Control System: The output has no effect on the input. It's a simple "do this" system.

Example: A toaster. You set the timer (input), and it toasts the bread for that amount of time, regardless of whether the bread is toasted enough (output). There's no feedback.

Closed-Loop Control System: The output does affect the input. It uses feedback to maintain a desired condition.

Example: A geyser (water heater). A thermostat (sensor) measures the water temperature (output). If the temperature is too low, the thermostat turns on the heating element (actuator) until the desired temperature is reached. Once the temperature is reached, the thermostat turns off the heating element. This is feedback.

Components of a Control System: Sensor: Detects a change in the environment (e.g., temperature, light, pressure).

Controller: Processes the information from the sensor and decides what action to take (e.g., a microchip, a thermostat).

Actuator: Performs the action (e.g., a motor, a valve, a heating element). Guided Practice (With Solutions)

Question 1: A gear train has a driving gear with 15 teeth and a driven gear with 45 teeth. What is the gear ratio, and what does this tell you about the speed and torque of the driven gear compared to the driving gear?

Solution: Gear Ratio = Number of teeth on driven gear / Number of teeth on driving gear = 45 / 15 = 3 The gear ratio is 3.