Science Fenns K-5Grades K to 5
A child holding a small cardboard bridge model beside a cup of washers used as test weights.

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Models and Designs

Build a prototype. Test it. Judge it.

Lesson 5 of 41

Try this first

Before we start

Today’s story

Models and Designs

Engineers do not build a bridge and hope for the best. They build a small one first. A model lets them test, fix and test again.

Every model has strengths and limits. A paper bridge shows how shape affects strength. It cannot show what a real river does to steel. Knowing the limits is part of using a model well.

Today you build a prototype, test it and judge the design. A paper bridge is all you need. Here are the steps.

See it

Five steps. Every time.

    Hands on

    Paper Bridge Test

    30 minutes · Do this with a grown-up nearby

    You need

    • Two stacks of books
    • Sheets of paper
    • Tape
    • Small cup
    • Pennies or washers
    • Ruler

    Do this

    1. Set two book stacks 15 centimeters apart.
    2. Make a bridge from one sheet of paper. Lay it across.
    3. Set the cup in the middle and add pennies one at a time.
    4. Count how many pennies it holds before it sags or falls.
    5. Fold the paper into an accordion shape and test again.
    6. Compare the counts.
    Write or draw: A table with design, pennies held and notes.

    Watch for: Notice how folding changes the strength. What would a real bridge need that your model lacks?

    Safety: Keep your hands clear of falling books.

    Same every time

    Three rules

    Rule 1Test, then improve

    Failure shows you what to fix.

    Rule 2Know your model's limits

    A model is a simplified copy.

    Rule 3Compare designs with numbers

    Numbers make it fair.

    Grandpa Fenn’s notebook

    Say it like a scientist

    model
    A copy or picture used to understand something.
    prototype
    A first version built to test an idea.
    design
    A plan for solving a problem.
    limit
    A place where a model stops being accurate.

    Poppy’s quiz

    Six questions

    Tap an answer. Green means right. You will see why.

    Talk about it

    Say it out loud

    • What evidence shows your design works?
    • What have you seen built from a small plan, like a doghouse or a barn?
    Did you know

    Engineers test bridge models in wind tunnels and water tanks to study forces before building the real thing.

    Someone who does this

    A civil engineer designs bridges and roads and tests plans before anything is built.

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