Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Identify and describe four main types of forces.
• Explain how materials and design affect strength.
• Analyse simple structures to find forces and weak points.
• Design a simple structure to withstand a specific force.

Lesson summary

This week, we delve deeper into the fascinating world of structures and how forces affect them. We’ll build upon last week's introduction to structures, focusing specifically on understanding the different types of forces acting on structures and how the strength of a structure relates to its ability to withstand those forces. Understanding these principles is crucial for anyone involved in building, designing, or even just observing the world around them. From the houses we live in to the bridges we cross, everything is a structure that must withstand various forces.

Lesson notes

Forces Acting on Structures: Forces are pushes or pulls that can cause an object to change its shape, speed, or direction. When it comes to structures, understanding the different types of forces is paramount.

Tension: Tension is a force that stretches or pulls on a material. Imagine a rope being pulled tight. The force within the rope is tension. In a building, tension can be present in suspension cables of a bridge or in the steel rods used to reinforce concrete. A real-world example is the cables supporting the Chapman's Peak Drive in Cape Town. These cables are under immense tension.

Compression: Compression is the opposite of tension; it squeezes or pushes on a material. Think of a stack of books; the bottom books are under compression due to the weight of the books above them. In a building, the walls and columns are primarily under compression, supporting the weight of the roof and upper floors. The foundation of a building is also under considerable compression.

Shear: Shear is a force that causes one part of a material to slide past another part. Imagine cutting paper with scissors; the force applied by the blades is shear force. Shear forces are often present in joints and connections within a structure. An example is where a beam rests on a pillar; the force trying to push the beam off the pillar is a shear force. Earthquakes can cause significant shear forces on buildings.

Torsion: Torsion is a force that twists a material. Imagine twisting a wet cloth to wring out the water; you are applying torsion. Torsion is often present in axles and shafts that rotate, but it can also affect buildings, especially tall ones, due to wind forces.

Strength of Structures: The strength of a structure refers to its ability to withstand forces without breaking or collapsing. Several factors influence the strength of a structure: Material: Different materials have different strengths. Steel, for example, is very strong in both tension and compression, while concrete is strong in compression but weak in tension. Wood is strong in tension along the grain, but weaker across the grain. Materials commonly used in South Africa, such as corrugated iron for roofing in informal settlements, have strengths and weaknesses that need to be considered.

Shape: The shape of a structure significantly affects its strength. Arches and domes, for instance, are very strong in compression. Triangles are also exceptionally strong shapes because they distribute forces evenly. Think of the A-frame houses often found in mountainous regions; the triangular shape is excellent at withstanding snow loads.

Design: The overall design of a structure is crucial for its strength. A well-designed structure will distribute forces evenly throughout the structure, minimizing stress on any particular point. Engineers use computer simulations to test designs and identify potential weaknesses. Consider the design of a simple bridge; the placement of supports and the shape of the bridge deck all contribute to its overall strength.

Connections: The way different parts of a structure are connected is critical. Weak connections can be points of failure. Using strong fasteners like bolts and welds is essential. In many rural areas of South Africa, simpler methods like lashing with rope are used for constructing temporary structures. Understanding the limitations of these connections is vital.

Worked example

Example: Wooden Beam under Compression: A wooden beam supports a portion of a roof. The weight of the roof exerts a compressive force of 5000 N (Newtons) on the beam. If the beam's cross-sectional area is 0.01 m 2 , calculate the compressive stress on the beam.

Solution: Stress is calculated as Force / Area.

Stress = 5000 N / 0.01 m 2 = 500,000 N/m 2 (Pascals). This tells us how much force is being distributed over each square meter of the beam's cross-section. We need to know the wood's compressive strength to determine if the beam will fail.

Example: Tension in a Rope: A rope is used to lift a bucket of water weighing 200 N. What is the tension in the rope?

Solution: The tension in the rope is equal to the weight of the bucket.

Tension = 200 N. The rope needs to be able to withstand at least 200 N of tensile force without breaking.

Example: Identifying Forces on a Bridge: Consider a simple beam bridge. When a car drives across it, what forces are acting on the bridge?

Solution: The bridge experiences several forces. The weight of the car and the bridge itself exerts a downward force, causing compression in the upper part of the beam and tension in the lower part. The supports at either end exert an upward force to counteract the weight. There's also shear force near the supports where the beam is trying to slide off.

Guided Practice (With Solutions)

Question: A brick wall is supporting a concrete roof. Identify the primary force acting on the brick wall.