Teaching with Catapult Lab

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Catapult Lab is a table seen from the side. On it sits a catapult at its real proportions: the stacked craft-stick catapult (a base stick, a stack of 3 to 9 sticks, a top stick banded down at one end with a cup on the other) or a plastic spoon on an axle, pulled forward by bands and stopped by a cross-bar. Students press the cup down or pull the spoon back, by dragging it or with a slider, and let go. The throw plays in slow motion over a tape measure on the floor, beside a dashed arc for the same throw with no air.

Under the table, labels name the effort, the fulcrum and the load. The card after each throw gives the distance and two energy bars: where the stored energy went when it let go (to the projectile, still swinging the cup and stick, turned into heat in the bands) and in the flight (still moving when it landed, taken by the air). A throws table keeps every throw by version, one version for each different build.

One period: the classic catapult, then why

1. Build it (5 minutes) On the projector, the loose parts. Start building and Next step put it together the real way: five sticks stacked and banded, two banded at one end, the stack pushed in, the cap glued on. Or Skip to the built one.
2. The first throw (5 minutes) Each student drags the cap down until it sits on the base, about 23 mm, and lets go. The pom-pom goes high, back over the banded end, and lands at 0.39 m. The dashed arc lands at 0.65 m. Ask which arc is the real one, and why the other is longer.
3. Change one thing (15 minutes) A paper cupcake liner instead of the cap: about 0.78 m, the same pom-pom twice as far, because the heavy cap was keeping most of the push. A ping pong ball from the cap: it takes a bigger share and still lands short. More sticks in the stack: lower and farther, 1.48 m with nine. The guide names what changed each time, and says so when two things changed at once.
4. Until it snaps (5 minutes) Four bands at the banded end and a full press: the stick snaps at about 21 mm and is drawn broken. The card says the stick gave out long before the bands did. Put on a new stick.
5. The best angle (10 minutes) Find the best angle throws the same pom-pom from every stack. The best is 66 degrees with nine sticks, the lowest this catapult can throw. Then the spoon catapult: its cross-bar sets the angle, and with one band a pom-pom goes 1.19 m at 45 degrees and about 1.59 m at 30. Ask why 45 lost both times.

With a real catapult

The app is built to sit beside the real catapult, not replace it. Print the build gives one page: the catapult drawn at real size, a 100 mm ruler to check the printer printed at 100%, the parts, the steps, a cut-out press gauge to stand beside the cup so every press is the same, where the drawing lands, and a safety box. Build it, throw it five times from the same press, and measure from the banded end to where it first touches.

Type the distances under Your real one. The app says what share of the drawing's distance the real one reached, works out how much less of the push the real one gives the throw, and uses that number for the next throws of the same build, marked "Tuned to your real one". Distance and energy do not shrink by the same share, and the app says so. A real one usually falls short: the bands are a different brand, the glue, the base or the hand gives a little, the stick and cup shake. Setup can turn this off.

What students get wrong, and where the app answers it

What the app simplifies, on purpose

• The throw is side on: nothing drifts left or right, and there is no wind or spin.
• The stick is stiff and does not whip past its rest shape; the base does not slide. The stack sits 40 mm from the banded end and the cup 10 mm from the free end.
• Two numbers are estimates still to be measured: how hard the bands at the banded end hold it down, and that the bands give back 75% of what was stored. The strongest light-cup builds may throw faster on the screen than real ones do.
• The pom-pom, marshmallow and drag values other than the ping pong ball's are estimates; the help never calls them measured.
• The spoon catapult's bands hook over a front post above the axle, and the spoon pulls back to 80 degrees from upright at most.

Grades 4 and 5, middle school and high school

The sheet and the setup

The Sheet in the app's header asks four questions written for each level: why the dashed arc went farther, why a heavier ball took a bigger share and still went short, the best angle and why it was not 45, and how the real one compared. Hand in needs every question answered and says how many are still empty.

Setup picks which questions are on the sheet (a fifth asks about the lever), adds questions of your own, one a line, and turns Your real one on or off. Six challenges sit under the throws: Into the cup at 1.5 m, A meter with sticks, Land together (a ping pong ball and a pom-pom within 10 cm), Twice as far, Few sticks far throw (2 m on the screen with 8 sticks or fewer) and A steady real one (three real throws inside 20 cm).

Standards

No NGSS performance expectation at any grade includes the two-dimensional arc, so the app does not claim one for it. The engineering and energy standards fit.

• MS-ETS1-4: "Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved." MS-ETS1-1 to 3 for criteria, comparing versions and combining what worked.
• MS-PS3-5: energy transferred from the bands to the arm, the projectile and the air, shown without calculations, inside its boundary. MS-PS3-1 for a heavier projectile against a lighter one.
• 3-5-ETS1-1 to 3, 4-PS3-4 (stored energy used to cause motion) and 4-PS3-1 (faster means more energy) at grades 4 and 5.
• HS-PS3-3: a device that converts energy, judged by its output for a given input. State physics, AP Physics 1 and Algebra 1 quadratics for the arc itself.

More help

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