Right Material for the Job — Physics (Grade 7)

Define and explain Right Material for the Job in your own words
Use key terms such as material accurately
Apply what you have learned to new examples and questions
Avoid the common mistakes learners make with this topic

This lesson focuses on Right Material for the Job: choosing materials for everyday objects based on their properties.

Definition: Right Material for the Job

Choosing materials for everyday objects based on their properties.

Key ideas

Materials have properties we can feel and see

Run your hand over sandpaper and silk and the difference is obvious: rough versus smooth. Properties like hardness, shininess and flexibility help us describe materials precisely. Scientists group materials by these properties to decide what each one is good for.

Some materials let water or light through, others block them

A sponge soaks up water while a plastic bag keeps it out — one is absorbent and the other is waterproof. Clear glass is transparent, frosted glass is translucent, and wood is opaque. These blocking and letting-through properties decide how we use each material.

Key term — material: What something is made from, such as wood, metal, plastic, glass, fabric or rock.

Which material for a raincoat?

Priya needs a material for a raincoat. She tests three: cotton fabric, plastic sheet and paper.

The raincoat must keep water out, so the material must be waterproof. Cotton soaks up water — it is absorbent, not waterproof. Paper tears and soaks through — no good either. The plastic sheet keeps water out and bends easily.

Answer: The plastic sheet — it is waterproof and flexible, the two properties a raincoat needs.

Common mistakes

Saying 'material' when you mean 'object' The object is the chair; the material is the wood it is made from.
Thinking glass is always transparent Frosted or stained glass is translucent — it lets some light through but you cannot see clearly through it.

Practice

Sort these into natural and man-made materials: wood, plastic, cotton, glass.

Natural materials come from plants, animals or rocks.

Natural: wood, cotton. Man-made: plastic, glass.

Name one transparent material and one opaque material.

Think of windows and walls.

Transparent: glass or clear plastic. Opaque: wood, metal or brick.

A material lets some light through but you cannot see through it clearly. What word describes it?

It sits between transparent and opaque.

Translucent.

Is a wooden spoon hard or soft? How could you test it?

Try scratching or bending it.

Hard — you cannot dent it with a fingernail or bend it easily.

Quick check

Right Material for the Job — quick check

Which of these best defines "material"?

What something is made from, such as wood, metal, plastic, glass, fabric or rock.

Give one property that makes metal good for saucepans.

Metal conducts heat well, so heat from the cooker spreads quickly through the pan. It is also strong and hard.

Why are umbrellas not made of paper?

Paper is not waterproof — it soaks up water and tears, so it would fail as an umbrella.

Key takeaways

Right Material for the Job: choosing materials for everyday objects based on their properties.
Materials have properties we can feel and see: Run your hand over sandpaper and silk and the difference is obvious: rough versus smooth.
property: Something you can observe about a material, such as whether it is hard, soft, shiny, rough or bendy.
Watch out for: saying 'material' when you mean 'object'

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