Science · Grade 7 · Form and Function of Structures · Term 4

Material Properties and Selection

Exploring how the properties of different materials (strength, flexibility, density) influence structural design.

• Ontario Curriculum Expectations
• MS-ETS1-3

About This Topic

In Grade 7 Science, material properties and selection teach students to evaluate strength, flexibility, density, durability, and other traits of materials like steel, wood, concrete, and plastics. They compare steel for high tensile strength in bridges, wood for flexibility and ease of shaping in frames, and concrete for compression resistance in foundations. Students analyze how these properties influence structure performance and justify choices for designs such as lightweight bridges that support heavy loads over gaps. This work meets Ontario curriculum expectations for the Form and Function of Structures unit.

Students apply engineering practices from MS-ETS1-3 by defining design criteria and constraints, including cost and sustainability. They recognize trade-offs, such as steel's strength versus its weight, and how combining materials optimizes outcomes. Class discussions reinforce that real-world structures succeed through informed selection.

Active learning excels with this topic. Students gain lasting insight by testing beams under weights, bending strips, or floating models in pairs. These experiences turn abstract properties into observable results, spark design iteration, and build confidence in engineering decisions.

Key Questions

1. Compare the suitability of steel, wood, and concrete for different structural applications.
2. Analyze how material properties influence the overall strength of a structure.
3. Justify the choice of specific materials for building a lightweight, strong bridge.

Learning Objectives

• Analyze the relationship between material properties (strength, flexibility, density) and their suitability for specific structural components.
• Compare the advantages and disadvantages of steel, wood, and concrete for building bridges and foundations.
• Evaluate design choices for a lightweight, strong bridge, justifying material selection based on property analysis.
• Explain how material properties influence the overall structural integrity and load-bearing capacity of a design.

Before You Start

Properties of Objects and Materials

Why: Students need foundational knowledge of basic material properties like hardness and texture before exploring more complex properties like strength and flexibility.

Forces Acting on Structures

Why: Understanding basic forces such as tension and compression is essential for analyzing how material properties resist these forces in structures.

Key Vocabulary

Tensile Strength	A material's ability to withstand pulling forces without breaking. Steel has high tensile strength, making it good for bridge cables.
Compressive Strength	A material's ability to withstand squeezing forces without deforming or breaking. Concrete excels in compressive strength, ideal for foundations.
Flexibility	A material's ability to bend or deform without breaking. Wood's flexibility allows it to be shaped and used in framing.
Density	The mass of a material per unit volume. Lower density materials are lighter for their size, important for lightweight structures.

Watch Out for These Misconceptions

Common Misconception

The strongest material works best for every structure.

What to Teach Instead

Strength matters, but flexibility prevents snapping under dynamic loads, and low density reduces total weight. Testing different beams in stations shows trade-offs clearly. Peer reviews during redesigns help students prioritize properties based on specific criteria.

Common Misconception

Wood always outperforms steel due to lightness.

What to Teach Instead

Wood flexes well but lacks steel's tensile strength for long spans. Hands-on bending and loading tests reveal limits. Group prototypes failing under weight prompt discussions on hybrid designs.

Common Misconception

Density only affects floating objects.

What to Teach Instead

High density increases foundation mass for stability but raises transport costs. Float tests with structure models demonstrate this. Collaborative data sharing connects density to land-based designs.

Active Learning Ideas

Testing Stations: Property Rotations

Prepare five stations: strength (drop weights on material spans), flexibility (hang weights on cantilever beams), density (submerge samples and measure displacement), durability (scratch or soak samples), conductivity (heat and touch). Small groups rotate every 7 minutes, record quantitative data like max load, and note observations.

45 min·Small Groups

Bridge Build Challenge

Provide popsicle sticks, straws, tape, and string. Pairs design a 40 cm span bridge to hold the most books. Build prototypes, test loads, record failures, and redesign once based on property insights.

60 min·Pairs

Material Selection Debate

Assign materials to small groups. Each presents a case for using their material in a tower or bridge, citing tested properties. Class votes on best choice after Q&A.

30 min·Small Groups

Density Sink-or-Float Structures

Individuals build small boats from foil, wood scraps, clay. Test buoyancy by adding cargo. Calculate density ratios and discuss applications for floating bridges.

25 min·Individual

Real-World Connections

• Civil engineers select materials like reinforced concrete and high-strength steel for skyscrapers in earthquake-prone regions, considering both compressive and tensile strength to ensure safety.
• Bridge designers choose between steel trusses for long spans requiring high tensile strength or concrete arch bridges for shorter spans where compressive strength is key.
• Architects and builders in cold climates use wood for framing due to its workability and insulating properties, while using concrete for foundations that resist frost heave.

Assessment Ideas

Quick Check

Present students with three structural scenarios: a bridge support column, a bridge deck, and a suspension bridge cable. Ask them to select the best material (steel, wood, or concrete) for each and write one sentence explaining their choice based on material properties.

Discussion Prompt

Pose the question: 'If you were designing a playground swing set, what material would you choose for the frame and what for the seat, and why?' Facilitate a class discussion where students justify their choices using terms like strength, flexibility, and durability.

Exit Ticket

Students receive a card with a material (e.g., steel, wood, concrete). They must write down two structural applications where this material is commonly used and one key property that makes it suitable for those uses.

Frequently Asked Questions

How to teach material properties for structures in Grade 7 Ontario Science?

Start with familiar examples like bridges, then use simple tests for strength, flexibility, density. Provide kits with wood, foil, plastic straws. Guide students to compare data tables and justify choices for designs. Link to real structures via photos or videos for context. This builds systematic evaluation skills over 4-5 lessons.

What materials work best for Grade 7 bridge building activities?

Popsicle sticks offer wood-like flexibility, straws mimic lightweight trusses, tape joins without adding bulk, foil tests waterproofing. Avoid glue for quick builds; it hides property effects. Scale to classroom spans of 30-50 cm. Emphasize iteration after failure tests to maximize load capacity.

How do material properties affect structure strength?

Strength combines compression resistance like concrete, tension handling like steel cables, shear resistance from layered wood. Density influences stability without excess weight. Flexibility absorbs vibrations. Students quantify via tests: measure deflection under 1 kg increments. Composites like reinforced concrete optimize multiple properties.

Why use active learning for material properties and selection?

Hands-on tests let students observe strength snap a straw or density sink foil directly, far beyond diagrams. Small group rotations ensure all participate, while redesign challenges teach iteration. Data collection and debates connect personal results to engineering principles, boosting retention and problem-solving confidence over passive methods.

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