# Paper Bridge Challenge — Strength per Gram, Tested in Class Source: https://decka.dev/classroom/physics/paper-bridge-challenge Teams load the paper bridges they built until each one breaks. Every result lands on one chart, the bridge's own mass across and the load it held up, under lines of ten to a hundred times its own weight, so the room can see which bridge held the most per gram. ## What it teaches Students compare strength against weight rather than strength alone: the load a bridge held divided by its own mass, read as a slope on the room's own chart, where the winner is the bridge on the steepest line, whether or not it is the heaviest. They test whether strength per gram comes from where the paper goes or from how much of it there is, and connect where their bridges broke to the squeezed top and the stretched bottom of a loaded span. ## A paper bridge challenge measured per gram Teams build their bridges before the lesson, all to span the same gap between two tables. In class, each team claims its bridge number on its phones and records three things on one shared team record: the bridge's mass from a kitchen scale, whether the design uses triangles, and the team's guess of how much it will hold. Anyone on the team can fix a typo until the weigh-in closes. The bridges go on show and everyone walks the gallery, then bets on the one that will hold the most for its own weight. The bets stay sealed until testing ends, so nobody herds onto an early result. Then each bridge is loaded at mid-span, sand poured slowly into a cup on a string until it fails, and its team records the total load it held and where it broke first. Each test lands on the projector's chart as its team records it: across, the bridge's own mass; up, the load it held. When testing closes, every point becomes the bridge it was, with triangles or without, and the sealed bets open onto the bridges they backed. Lines from the corner mark ten, twenty-five, fifty and a hundred times its own weight, so a bridge on a steeper line held more per gram, whatever it weighed, and the bridge on the steepest is ringed. Next, the rig counts where the bridges broke first, once per bridge, with a bridge that was never tested counted apart rather than as a zero. The screen shows no bridge numbers: the points are the room's, not a leaderboard. Each phone shows its own team's mass, load, how many times its own weight it held and where that placed it, and whether its own bet called the winner. Try the screen-and-phone preview: the phone is yours, on Bridge 4, and fictional classmates weigh, bet and test through the same runtime as the lesson. ## Classroom debrief Which bridge sits on the steepest line? Is it the heaviest? Why not? Most bridges broke first in one place. What does that say about where to spend your paper? The top of a loaded bridge is squeezed and the bottom is stretched. Which did our paper handle better? Before testing, most of the room bet on one bridge. What were we judging by? If you rebuilt with the same paper, what one change would you make, and which line would it reach? ## Paper bridge challenge questions, answered ### What do I need besides phones? A kitchen scale that reads to the gram, a cup on a string, sand or coins to load it, and two tables set a fixed gap apart. The gap is 30 cm unless you change it in the lesson's settings; the screen's test rig names it, so build to the gap you set. ### How do we load the bridges safely? Pour the sand slowly, so each bridge fails under a load its team can read, and keep feet and hands out from under the cup: when a bridge gives way, the cup and everything in it drops. ### How big are the teams? Up to five people share a bridge, and the lesson plans for four. Each phone joins with its team's bridge number, and a phone that gives a full bridge's number is asked to check it with you. Teams form when the weigh-in opens; anyone who joins after that bets and answers with the room but belongs to no team. ### Who records the results? Each team keeps one shared record for its bridge. One person enters it on their phone, and anyone on the team can correct it until that step closes; a change made from an outdated screen is refused rather than overwriting a teammate's. ### Are bridge numbers shown on the projector? No. The chart and the rig show every tested bridge without its number, and the bets open onto the bridges they backed without numbers too. Every test happens in front of the room, but a list of numbers would turn a lesson about design into a ranking of teams; each phone shows only its own team's results and place. ### Why compare per gram rather than the heaviest load? A bridge with twice the paper can hold more and still be the weaker design. Dividing the load by the bridge's own mass compares how well each one used its material, which is what an engineer is paid to improve. ### How long does it take? Building happens before the lesson. In class, testing takes as long as your bridges do: you move each step on when the room is ready, and no step runs on a timer. ## Run the lesson Present the boards in PowerPoint and Decka. Students scan a QR code or open the session link to join from their phone browsers.