A raincoat has to keep the rain out. But if it kept everything out, you'd be soaked in sweat inside it!
Can one material be perfect at two jobs at once? What do you think?
Every engineer who has ever designed a coat, a bag or a lunchbox has stood exactly where you're standing today: there's no perfect material, so you have to choose cleverly.
Today's words: property — something about a material we can test, like keeping water out or being light. Trade-off — getting more of one good property means giving up some of another.
Your group gets one everyday object. It has two jobs that fight each other. Which material is the best all-rounder for it?
Read your object brief card. It names your object's two jobs and shows you exactly how to test each one.
How to test each kind of job:
- Keeps rain out: drip water on and watch if it rolls off.
- Lets sweat escape: gently breathe through it and feel if air passes.
- Rigid (won't squash): rest a weight on it and see how much it bends.
- Light to carry: feel and compare the four by hand.
- Strong (won't tear): hold both ends and give a firm, steady pull.
- Soft (won't dig in): press it to your wrist or cheek.
Test each material for BOTH of your object's jobs. Score each one out of 3 for each job.
Predict first: which material do you think will be best all-round for your object? Write your guess before you start.
Let's take ONE group's four materials and put their two scores next to each other on the board, as a worked example we read together.
Now you can see at a glance that no single material wins both jobs. Watch for the pattern: the material that tops one column usually drops in the other.
Two scores for each material
Each group: name the material you recommend for your object, and say what you had to give up.
Listeners: challenge them! Ask what the losing material would have done better.
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