Lesson Notes By Weeks and Term v4 - JHS 2

Lesson Video

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Performance objectives

• Define what resistant materials are.
• Explain the basic properties like strength and durability.
• Identify examples of resistant materials used in Ghana.
• Understand how to choose the right material for a job.

Lesson summary

Welcome, learners! Today, we are going to explore the fascinating world of Resistant Materials. Look around you. The chair you are sitting on, the desk you are writing on, the windows in this classroom, and the tools a carpenter or a welder uses—all are made from materials chosen for a specific reason. These are often called resistant materials because they are strong and can withstand forces and wear. Understanding their properties is the first step to becoming a great designer, engineer, or artisan. It helps us choose the right material for the right job, whether we are building a house, making furniture, or creating beautiful metal gates.

Lesson notes

What are Resistant Materials?

Resistant materials are materials that can resist forces and wear without breaking or being easily damaged. They are durable and are often used to make products that need to last a long time and perform a specific function. They generally fall into three main categories: Woods: Examples: Odum, Wawa, Mahogany, Plywood. Metals: Examples: Steel, Aluminium, Copper, Iron. Plastics: Examples: PVC (polyvinyl chloride) for pipes, PET (polyethylene terephthalate) for water bottles, Acrylic.

Now, let's explore the *properties* that make these materials so useful. A property is a characteristic or quality of a material.

Basic Properties of Resistant Materials Hardness Definition: Hardness is the ability of a material to resist scratching, abrasion (wearing away), cutting, or indentation (denting). Explanation: A very hard material can scratch a softer material. Think about a diamond. A diamond is the hardest known natural material, which is why it can be used to cut glass. In our daily lives, a cutlass must have a hard cutting edge to cut through wood without becoming blunt quickly. Ghanaian Context: The terrazzo or polished concrete floor in many homes is hard, so it does not scratch easily when furniture is dragged on it. The blade of a farmer's hoe must be hard to dig into the soil without wearing away. Toughness Definition: Toughness is the ability of a material to absorb impact or sudden shocks without fracturing or breaking. Explanation: Toughness is different from hardness. A material can be hard but not tough (it is *brittle*). For example, a piece of glass is hard (difficult to scratch), but if you hit it with a stone, it shatters easily. It is not tough. A hammer head, on the other hand, is very tough. It can strike nails all day without breaking. Ghanaian Context: The plastic used to make a "Kufour gallon" is tough. You can drop it, and it will likely bounce or dent but not shatter. The handle of a pestle for pounding fufu must be tough to withstand the repeated impact. Strength Definition: Strength is the ability of a material to withstand an applied force without breaking or bending permanently. Explanation: There are different kinds of strength: Tensile Strength: Resisting a *pulling* force. A rope used for a tug-of-war needs high tensile strength. Compressive Strength: Resisting a *squeezing* or *pushing* force. A concrete pillar holding up a building needs high compressive strength. Shear Strength: Resisting a *sliding* or *cutting* force. The pin in a door hinge needs good shear strength. Ghanaian Context: The iron rods used in concrete pillars provide tensile strength, while the concrete itself provides compressive strength. This combination makes our buildings very strong. A wooden beam used for roofing must have enough strength to support the weight of the roofing sheets and resist the force of the wind. Elasticity Definition: Elasticity is the ability of a material to bend or stretch under force and then return to its original shape and size once the force is removed. Explanation: Think of a rubber band. You can stretch it, but when you let go, it snaps back to its original size. This is elasticity. A material that does not return to its original shape is said to have undergone *plastic deformation*. Ghanaian Context: The springs in a car's suspension system are elastic. They compress when the car goes over a bump and then return to their original shape, giving a smooth ride. The elastic in the waistband of your school shorts or skirt is another perfect example. Malleability Definition: Malleability is the ability of a material to be hammered, pressed, or rolled into thin sheets without breaking or cracking. Explanation: Most metals are malleable. This property allows us to shape them into useful forms. Imagine a blacksmith (tinsmith) hammering a flat piece of metal into the shape of a bowl or a scoop (a "bola"). Ghanaian Context: Aluminium is very malleable, which is why it is rolled into thin roofing sheets used on many houses in Ghana. Goldsmiths hammer gold into thin, intricate shapes to make beautiful jewelry because gold is highly malleable. Ductility Definition: Ductility is the ability of a material to be stretched or drawn into a thin wire. Explanation: This is a property similar to malleability but specifically about forming wires. Materials that are ductile can undergo large plastic deformation under tension. Ghanaian Context: Copper is a very ductile metal. This property is why it is used to make the thin electrical wires we use in our homes and for connecting our electrical appliances. The wires can be made very thin and long without breaking.