Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define and identify levers, pulleys, and gears.
• Calculate the mechanical advantage of a simple lever.
• Explain how simple mechanisms are used in everyday tasks and industries.
• Design a simple machine to solve a problem.

Lesson summary

This week, we're diving into the fascinating world of simple mechanisms and mechanical advantage. Simple mechanisms, like levers, pulleys, and gears, are all around us, making our lives easier and more efficient. From opening a cool drink with a bottle opener to lifting heavy building materials on a construction site, these mechanisms use clever engineering principles to amplify force and change direction. Understanding how they work is not just about technology; it's about problem-solving, critical thinking, and appreciating the ingenuity of design. Many industries in South Africa, from mining and agriculture to manufacturing and construction, rely heavily on these mechanisms.

Lesson notes

Let's break down the key concepts: Simple Mechanism: A simple mechanism is a basic device that changes the direction or magnitude of a force. They typically have few moving parts. These mechanisms use mechanical advantage to amplify force or increase distance.

Mechanical Advantage (MA): Mechanical advantage is the ratio of the output force (the force the machine exerts) to the input force (the force you apply). A mechanical advantage greater than 1 means the machine multiplies your force. A mechanical advantage less than 1 means the machine reduces your force but increases the distance or speed.

We calculate MA as: MA = Output Force / Input Force (MA = Load / Effort) MA = Distance of Effort / Distance of Load (Ideal MA)

Lever: A lever is a rigid bar that pivots around a fixed point called a fulcrum. There are three classes of levers, categorized by the relative positions of the fulcrum, load (resistance), and effort (force).

Class 1 Lever: Fulcrum is between the load and effort (e.g., seesaw, crowbar, pliers).

Examples found in South Africa: crowbars used for mining, pliers used by electricians.

Class 2 Lever: Load is between the fulcrum and the effort (e.g., wheelbarrow, bottle opener).

Examples found in South Africa: wheelbarrows for carrying sand and cement on construction sites, nutcrackers.

Class 3 Lever: Effort is between the fulcrum and the load (e.g., tweezers, fishing rod).

Examples found in South Africa: Tongs used for handling braai meat.

Lever Mechanical Advantage: The MA of a lever can be calculated using the following formula: MA = Distance from fulcrum to effort (Effort Arm) / Distance from fulcrum to load (Load Arm).

Pulley: A pulley is a wheel with a grooved rim around which a rope, cable, or belt passes. Pulleys are used to change the direction of a force and can also provide mechanical advantage.

Fixed Pulley: A single pulley attached to a fixed point. It changes the direction of the force but does not provide mechanical advantage (MA = 1).

Example: Raising a South African flag.

Movable Pulley: A pulley that moves with the load. It provides mechanical advantage (MA = 2).

Pulley System (Block and Tackle): A combination of fixed and movable pulleys. The mechanical advantage is equal to the number of rope segments supporting the load.

Example: Lifting heavy machinery in a Durban shipyard.

Gear: A gear is a rotating circular toothed wheel used to transmit rotary motion and torque. Gears can change the speed, torque, and direction of a rotating shaft.

Gear Ratio: The gear ratio is the ratio of the number of teeth on the driven gear (output gear) to the number of teeth on the driving gear (input gear). Gear Ratio = Number of teeth on driven gear / Number of teeth on driving gear If the gear ratio is greater than 1, the output gear rotates slower than the input gear, but with increased torque (force). If the gear ratio is less than 1, the output gear rotates faster than the input gear, but with decreased torque.

Worked example

Lever

Example: Imagine a builder in Johannesburg using a crowbar (Class 1 lever) to lift a heavy paving stone. The fulcrum is 0.2 meters from the stone (load), and the builder applies force 1.0 meters from the fulcrum (effort). If the stone weighs 200N, what force (effort) does the builder need to apply? What is the mechanical advantage?

Solution:*

MA = Effort Arm / Load Arm = 1.0m / 0.2m = 5

MA = Load / Effort => 5 = 200N / Effort

Effort = 200N / 5 = 40N

The builder only needs to apply 40N of force to lift the 200N stone. The mechanical advantage of 5 makes the task much easier.

Pulley

Example: A construction worker in Cape Town uses a pulley system with 3 rope segments supporting a load of bricks weighing 300N. How much force (effort) does the worker need to apply to lift the bricks, assuming ideal conditions? What is the mechanical advantage?

Solution:*

Since there are 3 rope segments supporting the load, the mechanical advantage is

3.

MA = Load / Effort => 3 = 300N / Effort

Effort = 300N / 3 = 100N

The worker needs to apply only 100N of force to lift the 300N bricks.

Gear

Example: A bicycle in Durban has a front gear with 48 teeth and a rear gear with 12 teeth. What is the gear ratio?

Solution:*

Gear Ratio = Number of teeth on driven gear / Number of teeth on driving gear = 12 / 48 = 0.25

The gear ratio is 0.

2

5. This means the rear wheel (driven gear) rotates 4 times faster than the pedals (driving gear), but with less torque, making it easier to pedal at a higher speed.

Guided Practice (With Solutions)

Question 1: A bottle opener is used to open a "Sparletta" cool drink bottle. The distance from the fulcrum to the bottle cap is 2 cm, and the distance from the fulcrum to where you apply force is 8 cm. What is the mechanical advantage of the bottle opener?