Using Materials
An engineer designing a cooking pot chooses metal because it conducts heat quickly and evenly. The same engineer puts a plastic handle on the pot because plastic is a terrible heat conductor — which means it stays cool enough to grab. The pot sits on a wooden cutting board because wood insulates the countertop from heat and resists scratching. Every material choice in engineering is a deliberate decision based on properties like strength, heat conductivity, weight, and cost.
Understanding Material Properties
Before running your experiments, understand the two properties you are testing:
Strength
Strength describes how much force a material can resist before it bends, breaks, or deforms permanently. Different materials resist force in different ways:
- Tensile strength — Resistance to being pulled apart (stretching)
- Compressive strength — Resistance to being crushed (squeezing)
- Flexural strength — Resistance to bending
For your experiments, you will primarily test flexural strength — how much a sample bends before breaking.
Heat Conductivity
Heat conductivity (also called thermal conductivity) measures how quickly heat flows through a material. Metals conduct heat very well — that is why a metal spoon left in hot soup gets too hot to touch. Wood and most plastics are poor heat conductors — they are insulators.
| Material | Heat Conductivity | Strength | Common Engineering Uses |
|---|---|---|---|
| Metal (steel) | Very high | Very high | Structural beams, tools, machinery, cookware |
| Metal (aluminum) | High | Moderate | Aircraft, beverage cans, heat sinks |
| Wood (oak) | Low | Moderate | Furniture, construction framing, flooring |
| Plastic (PVC) | Very low | Low to moderate | Pipes, insulation, packaging, housings |
| Plastic (nylon) | Very low | Moderate | Gears, bearings, cable ties |
Metal (steel)
- Heat Conductivity
- Very high
- Strength
- Very high
- Common Engineering Uses
- Structural beams, tools, machinery, cookware
Metal (aluminum)
- Heat Conductivity
- High
- Strength
- Moderate
- Common Engineering Uses
- Aircraft, beverage cans, heat sinks
Wood (oak)
- Heat Conductivity
- Low
- Strength
- Moderate
- Common Engineering Uses
- Furniture, construction framing, flooring
Plastic (PVC)
- Heat Conductivity
- Very low
- Strength
- Low to moderate
- Common Engineering Uses
- Pipes, insulation, packaging, housings
Plastic (nylon)
- Heat Conductivity
- Very low
- Strength
- Moderate
- Common Engineering Uses
- Gears, bearings, cable ties
Experiment 1: Testing Strength
Materials Needed
- Samples of wood, metal, and plastic of similar dimensions (e.g., thin strips about 6 inches long)
- Two supports of equal height (books, blocks, or cans)
- A small container (cup or bag) and weights (coins, marbles, or washers)
- A ruler
- A notebook to record results
Procedure
- Set up two supports about 4 inches apart
- Place each material sample as a bridge between the supports
- Hang or place the weight container in the center of the bridge
- Gradually add weight, recording how much the sample deflects (bends) at each increment
- Note the weight at which the sample bends permanently or breaks
- Repeat for all three materials
What to Observe
- Which material held the most weight before bending?
- Which material returned to its original shape after the weight was removed (elastic behavior)?
- Which material broke suddenly versus bending gradually?
Experiment 2: Testing Heat Conductivity
Materials Needed
- Samples of wood, metal, and plastic (spoons or rods of similar size work well)
- A cup of hot water (not boiling — hot tap water is sufficient and safer)
- A thermometer (optional but helpful)
- A timer
- Small dabs of butter or wax at the top end of each sample
Procedure
- Place a small dab of butter at the same point near the top of each sample
- Stand all three samples upright in a cup of hot water at the same time
- Start the timer
- Watch which butter dab melts first — this indicates the fastest heat conductor
- Record the time it takes for each butter dab to melt (or to begin melting after 5 minutes)
What to Observe
- The metal sample should conduct heat fastest — the butter melts quickly
- The plastic and wood samples should conduct heat much more slowly
- The order from fastest to slowest conductor should be: metal > wood > plastic (though wood and plastic are often close)
Why Engineers Care About Material Properties
The experiments you just ran are simplified versions of tests that materials engineers perform every day. When an engineer selects a material for a product, they consider dozens of properties:
- Strength-to-weight ratio — Aerospace engineers need materials that are both strong and light. Aluminum and carbon fiber composites win here.
- Corrosion resistance — Bridge engineers need materials that will not rust in rain and salt. Stainless steel and concrete are common choices.
- Cost — A material that is perfect in every way but costs 10 times more than an alternative may not be practical. Engineers balance performance with budget.
- Machinability — Can the material be easily cut, shaped, and joined? Some advanced materials are incredibly strong but nearly impossible to machine.
Discussing Your Results
When you talk with your counselor, be ready to explain:
- What you tested and how you set up each experiment
- Your results — which material was strongest? Which conducted heat best?
- Why the results make sense — connect your observations to the material’s structure (metals have tightly packed atoms that transmit both force and heat efficiently; plastics have long, flexible polymer chains that absorb force and block heat)
- Real-world applications — why is a frying pan metal but its handle is plastic? Why are house frames wood but the nails are steel?
