Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define 'simple mechanism' and give examples.
• Identify different types of simple machines.
• Explain and calculate 'mechanical advantage' for a lever.
• Understand how simple machines can be combined.

Lesson summary

This week, we're diving into the fascinating world of simple mechanisms and mechanical advantage! This topic is all about understanding how machines make our lives easier by amplifying our force. Imagine trying to lift a heavy rock to build a shack or fetch water from a deep well. Simple mechanisms are the tools that give us the "power" to do things we couldn't do alone. In South Africa, where resources might be limited, and human effort is crucial for survival and development, understanding simple machines is incredibly important. These mechanisms can be found in everything from basic tools used in agriculture to the more complex machinery used in construction.

Lesson notes

What is a Simple Mechanism? A simple mechanism is a basic machine that changes the direction or magnitude of a force. In simpler terms, it helps us to do work more easily. Simple mechanisms don't create energy; they just redistribute it. They allow us to use less force over a longer distance to achieve the same result, or use more force over a shorter distance, whatever is more practical. They achieve this by what's known as mechanical advantage.

Types of Simple Mechanisms: Lever: A rigid bar that pivots around a fixed point (fulcrum). Examples include seesaws, crowbars, and even your arm acting on your elbow joint.

Pulley: A wheel with a grooved rim around which a rope, cable, or belt passes. Used to lift or move objects.

Examples: hoisting flags, lifting buckets of water from a well, construction cranes.

Wheel and Axle: Consists of a wheel attached to a smaller rod (axle). Turning the wheel causes the axle to turn, and vice versa.

Examples: Steering wheel of a car, door knobs, gears.

Inclined Plane: A sloping surface used to raise objects. It requires less force to move an object up an inclined plane than to lift it straight up.

Examples: Ramps, stairs.

Wedge: A double inclined plane used to force things apart.

Examples: Axes, knives, chisels.

Screw: An inclined plane wrapped around a cylinder. Used to hold things together or to move objects.

Examples: Screws, bolts, jar lids.

Mechanical Advantage (MA): Mechanical advantage is a measure of how much a machine multiplies the force we apply. It is defined as the ratio of the output force (the force exerted by the machine) to the input force (the force we apply). ``` Mechanical Advantage (MA) = Output Force / Input Force ``` A mechanical advantage greater than 1 means the machine multiplies your force. A mechanical advantage less than 1 means the machine requires you to exert more force, but it might increase the distance or speed.

Levers in Detail: Levers are classified into three classes, based on the relative positions of the fulcrum, load (resistance), and effort (force applied).

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

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

Class 3 Lever: Effort is between the fulcrum and the load (e.g., tweezers, human arm). Calculating Mechanical Advantage of a Lever: For levers, the mechanical advantage can be calculated as: ``` MA = Distance from Fulcrum to Effort (Effort Arm) / Distance from Fulcrum to Load (Load Arm) ``` Example 1: A group of learners in Limpopo are trying to lift a heavy rock using a crowbar (a Class 1 lever) to clear land for planting. The fulcrum is placed 0.5 meters from the rock (load), and the learners apply force 2 meters from the fulcrum. What is the mechanical advantage? Effort Arm = 2 meters Load Arm = 0.5 meters MA = 2 / 0.5 = 4 This means the crowbar multiplies the learner's force by

4. If they apply a force of 50N, the crowbar exerts a force of 200N on the rock.

Example 2: Someone is using a wheelbarrow (a Class 2 lever) to carry soil. The distance from the wheel (fulcrum) to the center of the load is 0.4 meters, and the distance from the wheel to where they are holding the handles is 1.2 meters. What is the mechanical advantage? Effort Arm = 1.2 meters Load Arm = 0.4 meters MA = 1.2 / 0.4 = 3 Example 3: You are using a pair of tongs (Class 3 lever) to pick up a piece of braai meat. The distance from the pivot (fulcrum) to the meat (load) is 15 cm. You are applying your force 5cm from the pivot. Calculate the mechanical advantage. Effort Arm = 5 cm Load Arm = 15 cm MA = 5 / 15 = 0.33 (approximately) Notice that the mechanical advantage is less than

1. This type of lever doesn't multiply force; it increases distance and speed. Guided Practice (With Solutions)

Question 1: A builder in Durban needs to lift a heavy bag of cement using a pulley system. If the output force needed to lift the cement is 500N and he applies an input force of 250N, what is the mechanical advantage of the pulley system?

Solution: MA = Output Force / Input Force MA = 500N / 250N MA = 2

Commentary: This pulley system doubles the builder's force, making it easier to lift the heavy bag of cement.

Question 2: A child is using a seesaw. The child sits 1.5 meters from the fulcrum on one side. If an adult needs to balance the seesaw and sits 0.5 meters from the fulcrum on the other side, what is the mechanical advantage from the child's perspective?

Solution: MA = Effort Arm / Load Arm MA = 1.5 meters / 0.5 meters MA = 3

Commentary: From the child's perspective, the seesaw triples the force of the adult.

Question 3: A carpenter uses a ramp (inclined plane) to slide a heavy wooden beam into a truck. The ramp is 3 meters long, and the height of the truck bed is 1 meter. What is the theoretical mechanical advantage of the ramp?