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THE PHYSICAL EDITION

Folds and Bridges

Vince wants his duck to cross a paper bridge and his boat to pass underneath. Moving supports, folding paper and adding passengers turn one crossing into a playful design problem.

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Explore the science

A load can bend paperPages 1–2

Vince places his duck on a flat paper bridge. With his hands clear, the middle bends down. A flat sheet can bend under a toy between its supports. The picture shows one possible result, not a measured weight limit.

Ask together: What changes when Vince lets go?

Less paper left unsupportedPages 3–4

Vince brings the same end blocks closer. The duck stays up and the paper ends reach farther past the supports. Shortening the unsupported span can reduce bending under the same load. The longer overhangs help show what changed.

Ask together: Where does the paper reach beyond the blocks now?

A design has more than one jobPages 5–6

The boat is wider than the narrow opening. Vince moves the blocks apart again. A bridge can carry a toy yet leave too little room below. The boat gives Vince another requirement to consider.

Ask together: What is stopping the boat?

Shape changes how paper bendsPages 7–8

Vince folds one long edge upright, then the other, making an open channel. The same sheet gains a different cross section. Upright lengthwise edges can help it resist bending without adding more paper.

Ask together: What is different about the paper after the second fold?

Compare shapes at the same gapPages 9–10

The folded bridge returns to the wide opening. The duck stays up while Vince lifts both hands. The channel can resist bending better than the earlier flat sheet at the same span. Paper, folds and toy weight still matter.

Ask together: What stayed the same, and what changed?

Every structure has limitsPages 11–12

Vince adds red cubes beside the duck. The paper creases and the passengers drop into the sag. Holding one toy does not establish unlimited capacity. Enough added weight can permanently crease a paper bridge; the illustrated cube count is not a tested threshold.

Ask together: What do you notice about the paper after it bends?

Direct support takes up spacePages 13–14

Vince uses fresh flat paper and a broad middle block. The block supports the duck and cargo above but stops the boat below. A block in contact beneath the load can carry it through the paper. This is a new design with a fresh sheet and a new support, and it uses the passage the boat needs.

Ask together: What does the green block help, and what does it stop?

Combine discoveries to choose a designPages 15–16

Vince folds the fresh sheet, sets the green block and cargo aside, and sends his boat beneath the duck. The folded bridge carries the chosen light load while leaving the opening clear. Vince combines earlier discoveries to meet both needs.

Ask together: Which parts of the earlier bridges does Vince keep?

Sources and evidence

  1. A flat paper bridge can bend under a toy placed between its supports.

    File Card Bridges — Exploratorium Science-at-Home Team

    Depicted gentle sag, not fracture or permanent crease. Actual toy and paper not tested; no fixed weight or dimensions promised.

  2. Reducing the unsupported gap can reduce bending under the same central load.

    Beam Displacements — David Roylance, MIT, November30 2000

    Same flat sheet, same duck and equal-height supports. Overhang increases as supports approach; no hand holds the deck in result view. Qualitative elastic behavior, not a measured ratio.

  3. Folding the two long edges upright changes the paper cross section and can make a bridge resist bending better.

    Paper Bridge Design Challenge — Ben Finio, Science Buddies

    File Card Bridges — Exploratorium Science-at-Home Team

    A U-shaped channel is different from folding the sheet flat into layers. Folds run along the span. Compare same paper and duck at original support spacing. Not every fold makes every structure stronger.

  4. Adding enough load can crease and collapse a folded paper bridge; holding one toy does not establish unlimited capacity.

    Paper Bridge Design Challenge — Ben Finio, Science Buddies

    Depict cumulative loading followed by a sharp crease/collapse. Replace the damaged sheet explicitly; do not imply it springs back. Illustrated cubes are not a tested load rating or a guaranteed failure count.

  5. A block in direct contact beneath a load can support it through the paper while occupying the passage below.

    Beam Displacements — David Roylance, MIT, November30 2000

    The new flat sheet, middle block and loaded configuration are a redesigned bridge, not a controlled one-variable comparison with the failed folded sheet. Keep the full duck/cargo footprint directly over the broad support. Block contacts underside and floor. Boat obstruction is this design’s literal clearance constraint, not a universal property of supports.

  6. A useful bridge design must both carry its chosen load and leave the required passage clear.

    Paper Bridge Design Challenge — Ben Finio, Science Buddies

    An application of source engineering-design comparisons to this invented play problem, not an empirical test. Final scene changes both load and support deliberately; it is not called a controlled experiment.

Water and Wind

Water and Wind, pages 1–2: Wind moves the pinwheel.

Spread 1 / 12

Wind tickles my cheek. Round goes my pinwheel.

Explore the science

Wind & forcesPages 1–2

Wind moves the pinwheel. Wind is moving air. Vince feels it on his cheek and sees it push the pinwheel's angled blades.

Ask together: What tells you that the air is moving?

Currents & eddiesPages 3–4

Water passes the rock. A leaf circles behind it. Most water passes the rock; some circles back in an eddy. Laminar flow is orderly; turbulence is irregular, mixing motion. A swirl alone cannot distinguish them.

Ask together: How does the leaf move compared with the water passing the rock?

Waves & energyPages 5–6

Waves spread from the stream. The duck bobs. The falling stream makes waves that carry energy outward and make the toys bob. The stream's current also carries toys along. Bobbing and drifting are different motions.

Ask together: What moves up and down? What moves along the pond?

BuoyancyPages 7–8

A duck floats. A pebble sinks. Water pushes upward on both objects. This buoyancy balances the duck's weight, but not the pebble's. The pebble's splash makes the rings on the next spread.

Ask together: Where is each object compared with the surface?

When waves meetPages 9–10

Two wave patterns approach one another. When gentle waves overlap, their heights add, briefly raising or lowering the surface. This interference does not erase the waves; they continue through one another.

Ask together: What might happen where the two patterns meet?

Surface tensionPages 11–12

Tiny dents appear beneath the strider’s legs. Surface tension helps support the strider where its water-repellent legs dent the water. The duck floats mainly by displacing water.

Ask together: What do you notice where its legs meet the water?

Suction & pressurePages 13–14

The duck bubbles, fills and squirts. Squeeze underwater: air bubbles out. Release with the opening submerged: the elastic duck springs back, lowering pressure inside. The surrounding water pressure pushes water in. Squeeze above water: water exits through that same opening.

Ask together: What changes when Vince squeezes and then lets go?

Lift & gravityPages 15–16

A paper plane glides forward and down. The throw starts the plane moving. Airflow around its wings produces lift and turns air downward. Gravity pulls down; drag resists motion. In still air, the plane glides forward and gradually descends.

Ask together: Where might the plane go next?

ViscosityPages 17–18

Milk runs. Honey stretches into a thread. Honey's greater viscosity means greater resistance to flow. It makes a slow thread while milk runs readily. Temperature and pouring method also matter; this snack is an observation, not a controlled test.

Ask together: How do the milk and honey move differently?

Draining & swirlingPages 19–20

Foam curves inward toward the drain. Gravity drives water through the drain. Existing turning motion can become a curved inward flow, traced by foam. Here water travels to the outlet, unlike waves crossing the pond. Not every drain swirls.

Ask together: Can you follow the foam toward the drain?

Scientific observationPages 21–22

Vince sketches what he noticed. Vince records something he noticed. Drawings, marks or words let him return to an observation. He does not need all the answers.

Ask together: Which earlier picture matches one of his drawings?

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