Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define the six simple mechanisms
• Identify input and output forces
• Calculate mechanical advantage (MA)
• Explain how MA makes work easier
• Discuss the trade-off between force and distance

Lesson summary

This week, we delve into the fascinating world of simple mechanisms and mechanical advantage. Understanding how these work is crucial because simple mechanisms are the building blocks of almost every complex machine we use daily, from bicycles and cars to door handles and water pumps. In South Africa, especially in rural areas, understanding these mechanisms can be vital for maintaining essential equipment and even developing innovative solutions to local challenges. By grasping the principles of mechanical advantage, you'll be able to understand how to make work easier and more efficient, requiring less effort to achieve the same result.

Lesson notes

What are Simple Mechanisms? Simple mechanisms are basic devices that use force to perform work. They make work easier by changing the magnitude or direction of the force needed. They achieve this by trading force for distance (or vice versa). The six basic types of simple mechanisms are the lever, wheel and axle, pulley, inclined plane, wedge, and screw.

Lever: A rigid bar that pivots around a fixed point called a fulcrum. A lever amplifies an applied force (effort) to move a load. There are three classes of levers, distinguished by the relative positions of the fulcrum, effort, and load.

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

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

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

Example: Imagine using a crowbar (Class 1 lever) to lift a heavy rock in your garden. The rock is the load, the point where the crowbar rests on another rock is the fulcrum, and the force you apply to the end of the crowbar is the effort.

Wheel and Axle: Consists of a wheel attached to a smaller axle. Rotating the wheel requires less force than rotating the axle directly.

Example: Consider a steering wheel in a car or a tap (faucet). A large wheel (steering wheel) is connected to a smaller axle. The effort is applied to the larger wheel, making it easier to turn the smaller axle and control the car’s direction or flow of water.

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

Fixed Pulley: Changes the direction of force only (mechanical advantage = 1).

Movable Pulley: Provides mechanical advantage (mechanical advantage = number of rope segments supporting the load).

Example: A flagpole uses a fixed pulley to change the direction of force - you pull down to raise the flag. Construction workers often use systems of multiple pulleys (block and tackle) to lift heavy building materials. The more ropes supporting the load, the less force you need to pull.

Inclined Plane: A sloping surface used to raise or lower objects. It requires less force to move an object along an inclined plane than to lift it vertically.

Example: A ramp used to load goods onto a truck. It's easier to push a heavy box up a ramp than to lift it straight into the truck.

Wedge: A double inclined plane that is used to separate, split, or fasten objects. It converts a force applied to its wide end into a force perpendicular to its inclined surfaces.

Example: An axe used to chop wood, a knife used to cut food.

Screw: An inclined plane wrapped around a cylinder. It converts rotational motion into linear motion.

Example: A screw used to fasten pieces of wood together, a jack used to lift a car. Mechanical Advantage (MA) Mechanical advantage is the ratio of the output force (force exerted by the machine) to the input force (force you exert). It indicates how much a simple machine multiplies the force you apply. MA = Output Force / Input Force A mechanical advantage greater than 1 means the machine multiplies your force; you apply less force than the load's weight. A mechanical advantage less than 1 means the machine requires you to apply more force than the load's weight, but it may allow you to move the load faster or further.

Calculating Mechanical Advantage: Lever: MA = Length of Effort Arm / Length of Load Arm Effort Arm:* Distance from the fulcrum to the point where the effort is applied.

Load Arm:* Distance from the fulcrum to the point where the load is located.

Example: A Class 1 lever has an effort arm of 2 meters and a load arm of 0.5 meters. MA = 2 m / 0.5 m =

4. This means the lever multiplies your force by

4. Wheel and Axle: MA = Radius of Wheel / Radius of Axle

Example: A wheel has a radius of 30 cm, and the axle has a radius of 5 cm. MA = 30 cm / 5 cm =

6. This means the wheel and axle multiplies your force by

6. Pulley System: For a single fixed pulley, MA = 1 (only changes direction of force)* For a single movable pulley, MA = 2 For a pulley system (block and tackle), MA = number of rope segments supporting the load

Example: A pulley system has 4 rope segments supporting the load. The mechanical advantage is

4. Important

Note: Mechanical advantage comes at a cost. If a machine multiplies your force, you will have to move the input force over a greater distance. This is the principle of conservation of energy: work input = work output (ideally; in reality, some energy is lost due to friction). Guided Practice (With Solutions)

Question 1: A farmer is using a crowbar to lift a large rock. The distance from the fulcrum to the rock (load arm) is 0.3 meters, and the distance from the fulcrum to where the farmer applies force (effort arm) is 1.2 meters. What is the mechanical advantage of the crowbar?