Challenge 1
Empty a full magazine with no jams, and record how many bolts actually fed out of how many you loaded.
What you are building 3 min
The Chu Ko Nu: a repeating crossbow. A magazine of short beams sits on top, and a lever on the side does everything else.
There is no Brick in this model. No motor, no sensor, no cable, no program. That is not an omission — it is the lesson. Push the lever forward and back and the machine loads a bolt, draws the bow and looses it, in that order, every single time.
By the end of the lesson you will have measured how fast it can shoot and how far, and you will be able to say exactly where the “program” is stored.
Safety, before anything else. This machine launches a beam. Point it at a wall or a cardboard box, never at a person, an animal or a window. Everybody within two metres wears eye protection. One rule, no exceptions, all lesson.
In the real world 5 min
Where you have seen it
The chu ko nu is a Chinese repeating crossbow. Chinese tradition credits it to Zhuge Liang, the strategist of the Three Kingdoms period, though archaeologists have found repeating crossbows older than that — one from a Warring States tomb, centuries earlier.
Why it is built that way
An ordinary crossbow is powerful and slow. Loading one means bracing it against the ground, hauling the string back with both hands, seating a bolt, and aiming — twenty seconds if you are practised, and both hands are busy the whole time.
The chu ko nu gives away a great deal of that power and buys back speed. A magazine sits over the groove, and gravity drops the next bolt into place by itself. Working the lever forward and back draws the string, and at the end of the stroke a peg pushes up through a slot and lifts the string off — loosing the bolt.
One hand. One movement. Perhaps ten shots in fifteen seconds. It was a defensive weapon: not accurate, not powerful, but a wall of them could put an enormous number of bolts in the air very quickly.
What would go wrong without it
The order is not optional. Draw before the bolt has dropped and you shoot nothing. Release before the draw is complete and the bolt falls out of the groove. Every step depends on the one before, and there is no computer anywhere in the machine to check.
The order is not written down. It is built into the shape of the parts.
Before you build 1 min
There is no printed manual for this model — but the video above is a real build video, not a demonstration. It is Lau Mo’s assembly walkthrough for the 45544 set, so you can build straight from it. Pause it often; a video cannot be paged at your own speed the way a manual can.
Watch the finished mechanism run slowly, and more than once, before you start. You are looking for two moments: when the bolt drops into the groove, and when the peg lifts the string off. Those two are the whole machine.
The main concept — a sequence with no program 6 min
Here is what your machine does on every stroke of the lever. Read it as a program, because it is one.
- Load. The groove empties, and the bottom bolt in the magazine falls into it. Gravity does this — there is no part that pushes.
- Cock. The lever pulls the string back along the groove, behind the bolt.
- Release. At the very end of the stroke a peg rises through a slot, lifts the string off its catch, and the bolt goes.
Three steps, always in that order, never overlapping. Nothing checks. Nothing decides. There is no if, no wait until, and nothing that could fail to run.
Where the order actually lives
| In your EV3 programs | In this machine |
|---|---|
| Blocks in a stack, top to bottom | The travel of the lever, front to back |
A wait until that holds the next step back | A peg that is not yet under the slot, because the lever has not reached it |
| Changing the order means dragging blocks | Changing the order means rebuilding the machine |
| An error stops the program | An error jams the mechanism, loudly |
Both are real answers to the same problem, and the trade is worth naming. A program can be changed in ten seconds and can go wrong in ways nobody predicted. A mechanism cannot be changed without tools and cannot get the order wrong at all — the wrong order is physically impossible.
Why engineers still choose the shape
This is not a historical curiosity. A car’s ignition switch will not let you start the engine in gear. A microwave cannot run with the door open — not because the software checks, but because opening the door physically breaks the circuit. A lift door cannot open between floors.
In each case somebody decided the rule was too important to trust to a program. You will meet this again in Lesson 45, where the Archery Robot has to refuse to release an arrow it has not armed — and there you will have to build the interlock in blocks, because the machine will not do it for you.
Mechanisms without motors
Some models have no motors and no sensors at all. They are not lesser lessons — they are where the physics lives. A build that stores energy, changes a force, or turns one kind of motion into another explains why the powered models later need the gearing and the speeds they do.
The one bargain behind all of it
Gears, pulleys and levers look like three different topics and are three shapes of the same deal: whatever you gain in force, you pay for in distance. Halve the effort and you move twice as far; move half as far and you need twice the effort. No arrangement of anything gives you both, and recognising that saves a great deal of time hunting for one.
What a mechanism actually does is let you spend the same effort in a form you can manage — a little at a time instead of all at once, or all at once instead of a little at a time.
The nine ideas, and where each one lives
| Tutorial | The question it answers |
|---|---|
| Gearing up and gearing down | How do I trade turns for force? |
| Changing the direction of a turn | Why is it going backwards, and how do I drive an axle at 90°? |
| Pulley systems | How does a rope let a small motor lift a big load? |
| Levers — effort, load and pivot | Where should the pivot go? |
| Elastic and stored energy | How much further does pulling back twice as far actually get me? |
| Scissor mechanisms | How do I make something extend a long way — and why won’t it start? |
| Oscillatory motion | How do I turn going round into going back and forth? |
| Centre of gravity | Why does it keep falling over? |
| Biomimetic mechanisms | How does a machine walk without ever being told to? |
Friction — the one that is in all of them
Friction is what makes wheels grip and also what wastes effort. Every mesh, pin and rope turn loses a little to it, so a mechanism with more parts delivers less than the arithmetic promises. A build that binds — that is stiff to move by hand — is losing far more than that, and no amount of extra motor power fixes a mechanism that is rubbing.
Before measuring anything, turn the mechanism by hand. It should move smoothly and coast a little. If it does not, fix the build first; numbers taken off a binding mechanism describe the binding, not the idea you were trying to test.
How to investigate a mechanism
Since there is nothing to program, the work is measuring. Change one thing, keep everything else the same, and record what happened — pull the car back 5 cm, then 10, then 15, and measure how far each one travels. Three readings at each setting, not one, because a single run is not evidence.
That is genuinely how engineers test, and it is the same discipline that makes debugging a program work: change one thing at a time.
Elastic and stored energy
A stretched rubber band is a store. Pull it and you put energy in; let go and it gives the energy back. Nothing about that is obvious from looking at the band, and two things about it are genuinely surprising.
Potential, then kinetic
While it is stretched and held, the energy is not doing anything — it is potential energy, stored in the shape of the material. Release it and that becomes kinetic energy: the energy of something moving. A pull-back car is a machine for making that swap at a moment of your choosing.
The same store shows up all over a LEGO set:
- a rubber band stretched between two pins — a catapult, a launcher, a pull-back car;
- a torsion spring wound up — a mousetrap-style snap mechanism;
- a weight raised up — gravity is a store too, and a falling counterweight is a battery.
Double the pull is four times the energy
This is the first surprise. Energy stored in a spring or a band goes as the square of how far it is stretched — pull back twice as far and you have stored four times as much, not twice.
Which sounds like excellent news, and leads directly to the second surprise: the car does not go four times as far. Find out where the two part company.
Drag the car back, or use the arrow keys. Watch the two curves come apart.
50%of full stretch×1.0energy, against half stretch25distance travelled0%lost to spinning wheels
Every bit of the extra stretch is still turning into distance. Double this stretch and you store four times the energy — energy goes as the SQUARE of how far you pull.
Find the place where the green curve stops rising as fast as the orange one. That is the setting worth building for — and it is a number you measure, not one you can reason out.
Where the extra energy goes
Past a certain stretch the wheels can no longer grip hard enough to turn all that force into movement, so they spin. The energy still leaves the band — it goes into noise, heat and rubber marks on the table instead of into distance.
That ceiling is a property of your car on that surface: its weight, its tyres, how much of it is over the driven wheels. It is not something to look up. It is something to measure, and finding it is one of the most satisfying things a mechanical build has to offer.
Three things move the ceiling up, and all of them are build changes rather than bigger bands: more weight over the driving wheels, grippier tyres, and gearing the launch down so the same energy arrives as more force over more time.
Testing it properly
Change one thing at a time and run each setting three times, not once. Pull back 5 cm, then 10, then 15, measuring from the same starting line every time and recording every run. Three readings tell you whether a difference is real; one reading tells you nothing at all.
That is the discipline the whole of Mechanisms without motors is built around, and it is the same habit that makes debugging a program work.
Why it matters
Bows, clockwork, suspension springs and the recoil mechanism in a car door all store energy and give it back on demand. Storing energy slowly and releasing it quickly is how a small muscle — or a small motor — produces a large, brief force.
If the order really matters, build it into the shape. A mechanism cannot forget.
Stored energyFrom Level 1's Elastic Car — storing energy slowly and releasing it all at once.Show meHide
Elastic and stored energy
A stretched rubber band is a store. Pull it and you put energy in; let go and it gives the energy back. Nothing about that is obvious from looking at the band, and two things about it are genuinely surprising.
Potential, then kinetic
While it is stretched and held, the energy is not doing anything — it is potential energy, stored in the shape of the material. Release it and that becomes kinetic energy: the energy of something moving. A pull-back car is a machine for making that swap at a moment of your choosing.
The same store shows up all over a LEGO set:
- a rubber band stretched between two pins — a catapult, a launcher, a pull-back car;
- a torsion spring wound up — a mousetrap-style snap mechanism;
- a weight raised up — gravity is a store too, and a falling counterweight is a battery.
Double the pull is four times the energy
This is the first surprise. Energy stored in a spring or a band goes as the square of how far it is stretched — pull back twice as far and you have stored four times as much, not twice.
Which sounds like excellent news, and leads directly to the second surprise: the car does not go four times as far. Find out where the two part company.
Drag the car back, or use the arrow keys. Watch the two curves come apart.
50%of full stretch×1.0energy, against half stretch25distance travelled0%lost to spinning wheels
Every bit of the extra stretch is still turning into distance. Double this stretch and you store four times the energy — energy goes as the SQUARE of how far you pull.
Find the place where the green curve stops rising as fast as the orange one. That is the setting worth building for — and it is a number you measure, not one you can reason out.
Where the extra energy goes
Past a certain stretch the wheels can no longer grip hard enough to turn all that force into movement, so they spin. The energy still leaves the band — it goes into noise, heat and rubber marks on the table instead of into distance.
That ceiling is a property of your car on that surface: its weight, its tyres, how much of it is over the driven wheels. It is not something to look up. It is something to measure, and finding it is one of the most satisfying things a mechanical build has to offer.
Three things move the ceiling up, and all of them are build changes rather than bigger bands: more weight over the driving wheels, grippier tyres, and gearing the launch down so the same energy arrives as more force over more time.
Testing it properly
Change one thing at a time and run each setting three times, not once. Pull back 5 cm, then 10, then 15, measuring from the same starting line every time and recording every run. Three readings tell you whether a difference is real; one reading tells you nothing at all.
That is the discipline the whole of Mechanisms without motors is built around, and it is the same habit that makes debugging a program work.
Why it matters
Bows, clockwork, suspension springs and the recoil mechanism in a car door all store energy and give it back on demand. Storing energy slowly and releasing it quickly is how a small muscle — or a small motor — produces a large, brief force.
Say this back before moving on: “Load, cock, release — and the lever is the program.”
What’s in this build 4 min
Look at your model. Which electronic parts can you find? Look properly before you read on — this is the second model in the whole course where the honest answer is none.
| Part | What it is doing here |
|---|---|
| The magazine | Holds the bolts and feeds them by gravity. Its height sets how many shots you get; its width must suit exactly one beam. |
| The lever | The only input. Everything the machine does is a consequence of how far it has travelled. |
| The bow and its band | Stores the energy. Level 1’s Elastic Car in a new shape — slow in, fast out. |
| The release peg and slot | The cleverest part. It is a wait until made of plastic: it cannot lift the string until the lever has come far enough. |
| The groove | Guides the bolt. If it is too loose the bolt sits crooked and jams; too tight and friction eats the range. |
The Elastic Car was the first, in Level 1. Two models out of ninety-six with nothing electronic in them, and both are here to make the same point from different sides: the Elastic Car stores energy without a program, and this one stores a sequence without a program.
Ports — by their absence 4 min
Sensors go in ports 1, 2, 3, 4. Motors go in ports A, B, C, D. They are not interchangeable, and nothing will tell you politely if you swap them.
This model uses none of them, and the rule is still worth saying — because the interesting question is what it would take to add one.
| If you wanted to… | You would need | And it would go in |
|---|---|---|
| Count the shots fired | a Touch Sensor tripped by the lever at full stroke | port 1 |
| Know when the magazine is empty | a Colour Sensor looking into the magazine | port 3 |
| Work the lever automatically | a Large Motor and a crank | port A |
| Refuse to fire when nobody is clear | an Ultrasonic Sensor down the range | port 4 |
Every one of those is a real challenge for later today. Notice what they all have in common: they add knowing to a machine that already does the job perfectly well without it. The mechanism does not need a sensor. A person who wants to count, or record, or be warned does.
Check your own build now:
- Nothing to plug in — so instead, check the lever travels its full stroke freely, with no cable, hand or beam in its path.
- Check the magazine is loaded with beams that are all the same length. One odd beam jams the feed.
- Check the machine is pointed at your wall or box before you load it, not after.
Confirm the mechanism 2 min
Every other lesson checks the Brick’s screen here. There is no screen, so check the machine instead — and do it with the magazine empty.
- Empty the magazine. Work the lever through a full stroke, slowly. Watch the peg rise through the slot at the end. That is the release, and you should be able to point at it.
- Load one beam only. Work the lever slowly and watch it drop into the groove. Does it seat square?
- Fire that single beam at the box. One shot, watched from the side, tells you more than ten fired fast.
- Now load the magazine and count how many beams went in. Write the number down — step 10 needs it.
Step 1 is the equivalent of reading the port tiles. You are confirming the machine can do its job before you ask it to. On an EV3 model that means checking a screen; here it means watching a peg.
Make it move 10 min
There is no program in this lesson. Not a short one — none at all. The lever is the program, and your hand is the processor.
Run the machine properly and pay attention to what your hand is doing.
- Aim at the box. Check nobody is between you and it. Glasses on.
- Work the lever slowly for the first three shots. Say the three steps out loud as they happen: load, cock, release.
- Now empty the magazine at a comfortable speed. Count the shots as they go.
- Compare the number you fired with the number you loaded in step 8. If they do not match, some bolts did not feed — and finding out why is the most interesting thing in this lesson.
What success looks like: one continuous back-and-forth movement of the lever produces one shot per stroke, with no fiddling between shots, and the magazine empties without a jam.
Stop and notice what is not happening. You are not telling it when to load. You are not telling it when to release. You are supplying movement, and the machine is supplying order. Every EV3 model you have built so far has needed you to supply both.
If a stroke fires nothing, the bolt did not drop — check the magazine is not binding. If a bolt falls out of the groove, the release is coming too early: the peg is finding the slot before the draw is complete, which is a build problem and not a technique problem.
Change it and test 8 min
This is the body of the lesson, and it is a real measured experiment. Rule up a table with three columns — what I changed, shots that fed, distance — and fill it in as you go. One change at a time, and three shots per setting before you believe anything.
- Rate of fire. Time yourself emptying a full magazine slowly, then as fast as you can. Count how many actually fed each time. Fast is not free — where does it stop paying?
- Draw length. Work the lever only three-quarters of its stroke. Does it fire at all? Now full stroke. Measure how far the beam lands in each case, from the same spot on the floor.
- Band tension. Add a second rubber band. Measure the range again. Then count the jams — a stronger band draws faster and gives the bolt less time to seat.
- Magazine height. Load it half full, then completely full. Does a taller stack feed better or worse? Predict before you test: more weight pushing down, but more friction on the walls.
- Angle. Fire level, then tipped up by about 30°. Which goes further? Which feeds more reliably? These are two different questions and they may have opposite answers.
- Now break the order on purpose. Hold a finger over the magazine so no bolt can drop, and work the lever. The machine cocks and releases perfectly, and shoots nothing — and it has no idea. That is what Lesson 33’s nail gun dry-fire lockout exists to prevent.
Step 6 is the sting in the tail. The mechanism guarantees the order of the three steps and guarantees nothing about whether each one worked. A shape can enforce a sequence. Only a sensor can notice a failure. That is why you have spent forty lessons on sensors, and why this machine — for all its cleverness — is 2,000 years old and still cannot count.
Build the order into the shape. Build the checking into the program. Do not expect either one to do the other’s job.