Lesson Notes By Weeks and Term v5 - Grade 4

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Define 'structure', 'strength', and 'stability'
• Identify factors that make structures strong and stable
• Design and build a simple, stable model structure
• Explain why shapes like triangles are so strong

Lesson summary

This week, we're diving into the fascinating world of structures! Think about all the things around you – your school building, your house, bridges, even the furniture in your classroom. All of these are structures, and they need to be strong and stable to work properly and keep us safe. This topic is important because understanding how structures work helps us appreciate the engineering all around us and also helps us understand how to build things that last and don't fall down! In South Africa, we rely on strong and stable structures for everything from housing to transportation. Understanding these concepts is crucial for building a safe and sustainable future for our country.

Lesson notes

What is a Structure? A structure is anything that is built or made with a definite shape and size for a particular purpose. Structures can be natural, like a termite mound or a spider web, or man-made (artificial), like a building, a bridge, or a chair. A structure needs to be strong and stable to do its job properly.

Strength: Strength refers to a structure's ability to withstand forces (loads) acting on it without breaking or collapsing. A strong structure can support a heavy weight or resist a strong wind.

Think about a bridge: it needs to be strong enough to hold the weight of all the cars and trucks crossing it.

Stability: Stability is a structure's ability to remain upright and balanced, resisting being toppled over or falling down. A stable structure doesn't wobble or sway easily. Imagine trying to balance a tall tower of blocks – the more stable it is, the less likely it is to fall. A building needs to be stable to withstand wind and earthquakes.

Factors Affecting Strength and Stability: Several factors influence the strength and stability of a structure: Shape: Certain shapes are inherently stronger and more stable than others. Triangles are incredibly strong because they distribute forces evenly. Arches are also strong, allowing loads to be spread outwards. Think about the shape of a bridge arch or the triangular supports in a roof. Rectangles and squares are less stable on their own as they can be easily deformed into parallelograms, but become stronger if supported by cross braces that form triangles.

Materials: The materials used to build a structure play a huge role in its strength. Steel, concrete, wood, and brick are common building materials, each with different properties. Steel is very strong in tension (being pulled) and compression (being squeezed). Concrete is strong in compression but weak in tension. Wood is strong and flexible but can rot if not treated properly. Different materials are best suited for different parts of a structure.

Base: A wide, solid base provides greater stability. Think about a pyramid – its broad base makes it very stable. A narrow base is less stable, like trying to balance a pencil on its point. The wider the base, the lower the centre of gravity, and the more stable the structure.

Joining Techniques: How the different parts of a structure are connected is crucial. Weak joints can lead to failure, even if the individual materials are strong. Nails, screws, glue, welding, and interlocking are all ways to join materials together. In South African shack building, for instance, correctly overlapping corrugated iron sheets is crucial for weather resistance and stability.

Foundations: Foundations transfer the weight of the structure to the ground. A well-designed foundation prevents the structure from sinking or shifting. Foundations are especially important in areas with unstable soil or seismic activity.

Worked example

Example 1: The Bridge: Imagine a bridge spanning a river. The deck (the part cars drive on) is made of concrete reinforced with steel (rebar). The concrete is strong in compression, handling the weight of the cars. The steel rebar is strong in tension, preventing the concrete from cracking and breaking. The bridge supports might be arched to distribute the weight outwards to the ground. The foundations are deep and wide to anchor the bridge to the earth.

Example 2: A House: The walls of a house are often made of brick or concrete blocks. These materials are strong in compression, supporting the weight of the roof. The roof is often supported by wooden or steel trusses, which are triangular frames that distribute the weight evenly. The foundations are a concrete slab or strip footing that spread the load of the house over a wider area.

Example 3: A Classroom Desk: A classroom desk is usually made of wood or metal. The tabletop is supported by legs. The strength of the desk depends on the thickness of the materials and how well the legs are attached to the tabletop. A wobbly desk is unstable because the legs are not providing adequate support or the base is not even. Adding cross braces to the legs will improve stability.

Guided Practice (With Solutions)

Question 1:

Name three things that are considered structures. Explain why they are structures.

Solution:

A Tree: A tree is a natural structure. It has a definite shape (trunk, branches, leaves) and size, and its purpose is to grow and support its leaves to make food through photosynthesis.

A School Building: A school building is a man-made structure. It has a defined shape (walls, roof, floors) and size, and its purpose is to provide a safe and comfortable space for learning.

A Bird's Nest: A bird's nest is a natural structure. It has a defined shape (usually a bowl) and size, and its purpose is to provide a safe place for birds to lay their eggs and raise their young.