Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define key terms like structure, force, tension, and compression.
• Identify different forces acting on structures.
• Explain how a structure's shape relates to its strength.
• Understand how materials are chosen for structures.
• Learn about stability in structures.

Lesson summary

This week, we begin our exciting journey into the world of Structures! Structures are all around us – from the chair you're sitting on to the bridges you cross and the buildings you live in. Understanding how structures work is crucial because it allows us to design and build things that are safe, strong, and useful. In South Africa, where we are constantly building and improving our infrastructure, this knowledge is especially important. Consider the Gautrain, the bridges connecting our provinces, or even the RDP houses many South Africans call home - all are structures governed by these principles.

Lesson notes

Let's dive into the core concepts: Structure: A structure is something that is built or constructed with a definite size and shape for a particular purpose. Examples include buildings, bridges, furniture, and even the frame of a bicycle. In essence, it is anything that has been put together to support a load or resist forces.

Force: A force is a push or a pull that can cause an object to change its motion or shape. Forces are measured in Newtons (N). In the context of structures, forces can be external (acting on the structure from the outside) or internal (forces within the material of the structure).

Tension: Tension is a pulling force. Imagine pulling on a rope. The rope is under tension. In a structure, tension occurs when a force tries to stretch or lengthen the material. Cables on a suspension bridge are under tension. Think about washing lines in your yard - they are designed to withstand tension forces from the wet clothes hanging on them.

Compression: Compression is a pushing or squeezing force. Imagine stacking books on top of each other. The books at the bottom are under compression. In a structure, compression occurs when a force tries to shorten or squash the material. Pillars in a building are under compression. Think about the brick walls of your house bearing the weight of the roof – they are withstanding compression.

Shear: Shear is a force that acts parallel to a surface, causing one part of the material to slide past another part. Imagine cutting paper with scissors. The paper is experiencing shear forces. In structures, shear forces can occur in bolts connecting two pieces of metal, or in the soil beneath the foundations of a building during an earthquake.

Strength: Strength refers to the ability of a structure to withstand forces without breaking or deforming excessively. Different materials have different strengths. Steel is generally stronger than wood, and wood is stronger than paper. The shape of a structure also affects its strength.

Stability: Stability is the ability of a structure to remain upright and resist toppling over. A stable structure has a low center of gravity and a wide base.

Worked example

Example 1: A Wooden Chair

Forces: When you sit on a wooden chair, you exert a downward force (your weight) on the seat. The legs of the chair exert an upward force to support your weight.

Compression: The legs of the chair are under compression because they are being pushed down by your weight.

Tension: The seat of the chair, especially if it's a woven seat, might experience some tension as it stretches slightly to support your weight.

Shape and Strength: The legs are usually thicker than the seat because they need to withstand greater compressive forces. The shape of the legs (straight and vertical) also contributes to their strength.

Example 2: A Bridge

Forces: A bridge experiences various forces, including the weight of the bridge itself (dead load), the weight of vehicles crossing the bridge (live load), and wind forces.

Tension: Suspension bridges use cables to support the bridge deck. These cables are under significant tension.

Compression: The bridge's support pillars are under compression.

Shear: Shear forces can act on the joints connecting different parts of the bridge.

Shape and Strength: Bridges often use trusses (triangular frameworks) to distribute forces efficiently and increase their strength. The material (usually steel or concrete) is chosen for its high tensile and compressive strength.

Example 3: A Shack (Informal Housing)

Forces: A shack experiences wind forces, the weight of the roofing material (corrugated iron or plastic sheeting), and potentially the weight of items stored inside.

Tension: The wires holding the corrugated iron to the frame might be under tension.

Compression: The support poles are under compression.

Shear: Wind forces acting on the sides of the shack can create shear forces in the joints.

Shape and Strength: Shacks are often not very strong or stable because they are built with inexpensive materials and without careful engineering. This highlights the importance of proper structural design.

Guided Practice (With Solutions)

Question: Identify the main force acting on the following structure: A stack of bricks.

Solution: The main force acting on the stack of bricks is compression. The weight of the bricks above presses down on the bricks below, creating a compressive force.

Question: A rope is used to pull a heavy bucket of water from a well. What type of force is the rope experiencing?

Solution: The rope is experiencing tension. The weight of the bucket of water is pulling on the rope, creating a pulling force.

Question: A wooden beam is supporting the roof of a small shed. What type of force is the beam most likely experiencing? Explain why.

Solution: The beam is most likely experiencing compression (primarily) and bending, which introduces both tension and compression within the beam itself. The roof's weight pushes down on the beam, causing compression. The bending action causes tension on the bottom of the beam and compression on the top of the beam.

Question: Imagine you have a flimsy cardboard box. You can easily squash it down by pressing on the top and bottom. What type of force are you applying to the box? What could you do to the box to make it resist this force better?