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.
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.
More mechanics tutorials
- Mechanisms without motors — How to investigate a build with no electronics in it, and where each idea lives.
- Gearing up and gearing down — Trade turns for force, or force for turns — you cannot have both.
- Changing the direction of a turn — Reverse a turn, restore it with an idler, or send it round a 90° corner.
- Pulley systems — Fixed, movable and combined — how rope pulled trades against force needed.
- Levers — effort, load and pivot — Where the pivot sits decides the force you need and the distance you get.
- Scissor mechanisms — Crossed links that extend and retract, and why the last bit is the hardest.
- Oscillatory motion — Turning a rotation into a back-and-forth, and what sets the rhythm.
- Centre of gravity — Why a robot tips, and how to build one that does not.
- Biomimetic mechanisms — Linkages that copy how animals move — turning a rotation into a step.