Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify four simple mechanisms with South African examples.
• Define and calculate mechanical advantage for levers.
• Explain how simple mechanisms combine into complex machines.
• Understand the relationship between force, distance, and work.
• Design a simple machine to solve a real-world problem.

Lesson summary

This week, we'll be diving into the fascinating world of simple mechanisms and mechanical advantage. Simple mechanisms are the building blocks of many complex machines we use every day, from bicycles to cars. Understanding how they work allows us to appreciate the ingenuity behind these inventions and even create our own solutions to everyday problems. In South Africa, these mechanisms are crucial in various sectors, including agriculture, transportation, and construction. Imagine trying to lift heavy loads without levers or pull water from a well without a pulley – life would be much harder!

Lesson notes

What is a Mechanism? A mechanism is a device that changes movement or force to perform a task. Simple mechanisms are the basic building blocks of more complex machines. They help us to do work more easily by changing the direction or magnitude of a force. We call this mechanical advantage. Mechanical Advantage (MA) Mechanical advantage is a measure of how much a mechanism multiplies the force we apply. A mechanical advantage greater than 1 means the mechanism increases the force; a mechanical advantage less than 1 means the mechanism decreases the force (but increases the distance).

Formula: Mechanical Advantage (MA) = Output Force (Load) / Input Force (Effort) Also, for some mechanisms like levers, you can calculate MA by considering distances: MA = Distance of Effort / Distance of Load Types of Simple Mechanisms: Lever: A rigid bar that pivots on a fixed point called a fulcrum. Levers are classified into three classes depending on the relative positions of the fulcrum, load, and effort.

Class 1 Lever:* Fulcrum is between the effort and the load (e.g., seesaw, crowbar, pliers). MA can be greater than, less than, or equal to

1. Class 2 Lever:* Load is between the fulcrum and the effort (e.g., wheelbarrow, bottle opener, nutcracker). MA is always greater than

1. Class 3 Lever:* Effort is between the fulcrum and the load (e.g., tweezers, fishing rod, human arm). MA is always less than

1. Worked

Example: A person uses a crowbar (a Class 1 lever) to lift a rock. The rock weighs 200N (Load). The distance from the fulcrum to the rock is 0.5m (Load Distance), and the distance from the fulcrum to where the person applies force is 2m (Effort Distance). What is the MA and the effort force required?

Solution: MA = Effort Distance / Load Distance = 2m / 0.5m = 4 MA = Load / Effort => Effort = Load / MA = 200N / 4 = 50N This means the person only needs to apply a force of 50N to lift the 200N rock. The lever multiplies the force by a factor of

4. Pulley: A grooved wheel with a rope or cable running around it. Pulleys can change the direction of force and can also provide mechanical advantage.

Fixed Pulley:* Attached to a stationary object (e.g., a flagpole pulley). It changes the direction of force, but the MA is

1. Movable Pulley:* Attached to the load (e.g., a crane lifting a container). It provides mechanical advantage. The MA is equal to the number of rope segments supporting the load. Worked

Example: A construction worker in Durban uses a pulley system with 3 supporting ropes to lift a bag of cement weighing 150

N. What is the mechanical advantage, and how much effort force is required to lift the cement?

Solution: MA = Number of supporting ropes = 3 MA = Load / Effort => Effort = Load / MA = 150N / 3 = 50N The worker only needs to apply a force of 50N to lift the 150N bag of cement.

Wheel and Axle: A wheel attached to a smaller cylinder (axle). When the wheel turns, the axle also turns. The wheel and axle provides a MA and rotates 360 degrees. Examples include steering wheels, screwdrivers, and doorknobs. Worked

Example: A well uses a wheel with a radius of 30cm connected to an axle with a radius of 5cm. If the bucket of water being lifted weighs 25N, what is the MA and what force must be applied to the wheel to raise the bucket?

Solution: MA = Radius of Wheel / Radius of Axle = 30cm / 5cm = 6 MA = Load / Effort => Effort = Load / MA = 25N / 6 = 4.17N (approximately)

Inclined Plane: A sloping surface used to raise objects. It reduces the force required but increases the distance. Examples include ramps, slides, and hills. Worked

Example: A worker needs to load a 100N box onto a truck. They can either lift the box directly or use a ramp that is 3m long and 1m high. Calculate the MA of using the ramp.

Solution: MA = Length of Slope / Height of Slope = 3m / 1m = 3 This means it takes 3 times less effort to push the box up the ramp compared to lifting it straight up.

Combining Simple Mechanisms: Complex machines often combine multiple simple mechanisms to achieve more complicated tasks. For example, a bicycle uses levers (brakes), wheel and axle (wheels), and gears (which are a type of rotating lever) to move. Guided Practice (With Solutions)

Question 1: A builder in Cape Town uses a wheelbarrow to move a pile of bricks. The bricks weigh 300N. The distance from the wheel (fulcrum) to the center of the bricks (load) is 0.4m, and the distance from the wheel to where the builder applies force (effort) is 1.2m. Calculate the mechanical advantage of the wheelbarrow and the force the builder needs to apply.

Solution: This is a Class 2 lever. MA = Effort Distance / Load Distance = 1.2m / 0.4m = 3 Effort = Load / MA = 300N / 3 = 100N The builder needs to apply a force of 100

N. Question 2: A farmer in Limpopo uses a pulley system to lift water from a well. The bucket of water weighs 80N. The pulley system has 2 supporting ropes.