Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Define and differentiate between types of forces (tension, compression, etc.).
• Explain the concepts of strength and stability.
• Identify methods for strengthening structures.
• Analyse forces acting on simple structures.
• Understand how material properties influence strength.

Lesson summary

This week, we delve deeper into the fascinating world of structures, focusing specifically on the forces that act upon them and the measures we take to ensure their strength and stability. Understanding these principles is crucial not just for passing your Technology class, but also for comprehending the world around you. From the houses we live in to the bridges we cross, structures are all around us, and their safety depends on the application of sound engineering principles.

Lesson notes

Forces Acting on Structures: Tension: A pulling force that stretches or elongates a material. Imagine pulling on a rope – that's tension. In structures, cables and ropes are often subjected to tension. Think of the cables suspending the Bloukrans Bridge, a famous bridge in South Africa.

Compression: A pushing force that squeezes or shortens a material. Think of leaning on a wall – you are compressing it. Pillars and columns are designed to withstand compressive forces. The walls of a building experience compression due to the weight of the roof.

Shear: A force that causes one part of a material to slide past another part. Imagine using scissors to cut paper – that’s shear force. Rivets and bolts in structures are often subjected to shear forces. An example is the force acting on a bolt that is used to join two plates together.

Torsion: A twisting force. Imagine twisting a wet cloth to wring out the water – that's torsion. Screws and axles are often subjected to torsion. Think of a screw holding two pieces of wood together being tightened – that is torsion.

Bending: A force that causes a material to curve or deflect. Imagine placing a book on two supports and pushing down in the middle – that’s bending. Beams and shelves are designed to resist bending forces. A beam holding up the roof of a house is subjected to bending forces.

Strength and Stability: Strength: A structure's ability to withstand forces without breaking or failing. A strong structure can bear a large load without collapsing.

Stability: A structure's ability to maintain its shape and position without toppling over or collapsing. A stable structure will not easily tip over, even when subjected to external forces. A tripod is a very stable structure. A structure can be strong but not stable, or stable but not strong. For example, a very tall, thin tower made of strong steel might be strong enough to withstand the weight, but unstable and prone to toppling over in the wind.

Methods of Strengthening Structures: Triangulation: Using triangles in a structure's design makes it more rigid and resistant to deformation. Triangles are inherently stable shapes because their angles are fixed. Think of the steel trusses used in the roof of a large warehouse - they're often made of triangles.

Using Appropriate Materials: Different materials have different strengths and weaknesses. Steel is strong in both tension and compression, while concrete is strong in compression but weak in tension. Selecting the right material for the job is crucial. Using reinforced concrete (concrete with steel bars embedded in it) is a common way to improve a structure's resistance to both tension and compression.

Providing Support: Adding supports to a structure can help distribute forces and prevent it from collapsing. Supports can be in the form of columns, beams, or braces. Think of the supporting pillars under a bridge.

Material Properties & Strength: Tensile Strength: The maximum tensile stress a material can withstand before breaking.

Compressive Strength: The maximum compressive stress a material can withstand before crushing.

Yield Strength: The amount of stress a material can withstand before it begins to deform permanently.

Elasticity: The ability of a material to return to its original shape after a force is removed.

Ductility: The ability of a material to be stretched into a wire without breaking.

Hardness: The resistance of a material to scratching or indentation. Consider the corrugated iron used in many South African homes. While relatively lightweight and affordable, it has good tensile strength along its corrugations, making it resistant to tearing.

However, it has poor compressive strength, meaning it can easily be dented or crushed.

Worked example

Example 1: Analyzing a Chair: Imagine a simple wooden chair. When someone sits on it, the legs experience compression, the seat experiences bending, and the joints where the legs connect to the seat experience shear. To improve the chair's strength, you could use thicker wood for the legs and seat (increasing compressive and bending strength), and reinforce the joints with screws or glue (increasing shear strength).

Example 2: Strengthening a Cardboard Bridge: Let's say you're building a cardboard bridge. The cardboard tends to bend easily. How can you strengthen it? You could fold the cardboard into a "U" shape to create a beam, which is much stronger than a flat piece of cardboard due to increased resistance to bending. You could also add triangular supports underneath the bridge to further resist bending.

Guided Practice (With Solutions)

Question 1: Identify the primary force acting on the following structural elements: a) a rope supporting a swing, b) a pillar supporting a roof, c) a bolt holding two metal plates together.

Solution:

a) Tension (the rope is being pulled).

b) Compression (the pillar is being pushed).

c) Shear (the bolt is preventing the plates from sliding past each other).

Question 2: Explain how triangulation strengthens a structure. Give an example of a structure that uses triangulation.

Solution: Triangulation strengthens a structure because triangles are inherently stable shapes. Their angles are fixed, preventing deformation. A force applied to one side of a triangle is distributed along the other two sides, making it very rigid.

Examples include: radio masts, bridge trusses, and bicycle frames.

Question 3: A builder is constructing a wall. He can choose between clay bricks and concrete blocks. Concrete blocks are slightly more expensive but have significantly higher compressive strength. In what scenario might he choose the concrete blocks over the clay bricks?

Solution: The builder should choose concrete blocks if the wall is intended to support a very heavy load (e.g., a second story). The higher compressive strength of the concrete blocks will ensure that the wall can withstand the weight without collapsing. If the load is relatively light, the clay bricks might be sufficient, and the builder could save money.

Question 4: Describe how you could increase the stability of a tall, narrow tower made of building blocks.