A catapult and a trebuchet both store energy and then let it go all at once to throw something. A spring catapult stores it in a bent or stretched spring. A counterweight trebuchet stores it in a heavy weight lifted high. Both then use a lever arm to turn that stored energy into a fast swing, and the shot's speed and angle decide where it lands.
That makes them a brilliant hands-on lesson in energy, levers and aiming. Build one or both, then use the range game below to learn to predict where a shot will land.
Two ways to store energy
Stored energy is energy waiting to be used. Push the arm of a spring catapult down and you feel the spring push back: that push is energy you have put in. Lift the weight of a trebuchet and you have put energy in too, this time by raising something heavy against gravity. Let go, and the energy comes back out as motion. The stored energy idea page has more examples, from wind-up toys to bows.
Neither machine gives back more than you put in. A stronger spring or a heavier, higher weight means more energy to start with, and so a faster swing. Friction in the pins and the air around the arm take a share before the ammo leaves.
The Spring Catapult
The Spring Catapult is a level 2 build of about 25 minutes. Cock the arm, click the trigger, and a spring flings the ammo across the room. The arm is a lever, long on one side of the pivot and short on the other. A grey tooth holds the arm down until you pull the latch back. When the arm swings up it bangs into a stop bar, and the ammo keeps flying. That stop bar sets the release angle.
The Counterweight Trebuchet
The Counterweight Trebuchet is a level 3 build of about 40 minutes that uses the heavy battery box as its weight. The pivot sits near one end of the arm: the short side is 4 holes long, the long side 10. A small drop at the short end becomes a big, fast swing at the long end. Hanging links act as a hinge so the weight falls straight down instead of swinging out, a bush outside each link keeps the weight from sliding along its axle, and a trigger axle holds the arm until you pull it out. The build suggests adding a string sling at the tip, like real trebuchets used.
It lists the Worm-Gear Crane and the windmill as earlier builds, so it is a good second or third mechanism rather than a first one.
Predict, launch, score
Both machines decide two things about each shot: how fast it leaves and at what angle. Everything after that is gravity. Try it below. Set an angle and a speed, tap the floor where you think the shot will land, then launch. You score 3 points for landing on the cushion, and up to 2 more for a close prediction, over five shots.
Interactive: Launch range predictor. Set the launch angle and speed of an ideal projectile, predict where it lands and try to hit the target cushion.
Press "Sweep 15° to 75°" to draw five shots at the same speed. On flat ground, 45° goes furthest. Look at 30° and 60°: they land in the same place, but the 60° shot climbs higher and spends longer in the air. Now switch the release point to the raised arm and sweep again. The best angle drops below 45°, because the shot already starts above the floor.
Why real throws differ from the game
The game is an ideal model with made-up units, not a measurement of either build. Real shots differ in three main ways:
- Release height. A real arm lets go above the floor, which favours lower angles, as the raised-arm option shows.
- Air. Air slows a light shot, especially a fast one. It shortens the range and pushes the best angle lower still.
- The arm itself. Part of the stored energy goes into swinging the arm and shaking the frame, not into the shot. A lighter arm wastes less of it.
Levers behind the throw
Both machines use a lever: a bar turning on a pivot. Build the Balance See-Saw first (level 1, about 12 minutes) to see how distance from the pivot matters. Its instructions show one gear six holes out balancing one gear at two holes plus one gear at four. The lever classes page explains the three kinds of lever.
The Parking Barrier Gate (level 2, about 20 minutes) uses a counterweight the gentle way: a heavy wheel balances the long boom, so a tiny turn lifts it. Take the weight off and lifting gets hard. The trebuchet uses the same idea, except it lets the weight fall on purpose. The counterweight page has more.
A fair test for your real machine
- Mark the machine's position on the floor with tape.
- Fire five shots with the same ammo and the same setting.
- Mark each landing spot and measure the distances.
- Change one thing only, such as the counterweight or the ammo, and repeat.
Real shots scatter. Compare the middle value of each set of five rather than the single best shot. The fair-test steps in the pull-back car experiment work the same way here.
Check yourself
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- Where does a counterweight trebuchet store its energy before it fires?
- In the ideal launch model (flat ground, no air), which angle lands furthest?
- At the same speed, 30° and 60° shots on flat ground with no air…
- Why does a real throw usually go furthest at an angle below 45°?
