Get Cracking!

In February earlier this year, Yanhao, a few other people and myself attended a national physics competition (ooh I know, fancy) in London hosted by the Weizmann Institute of Science. This competition required the teams to design and build a safe which would be able to be cracked using various physics principles. For example, the ‘crackers’ may have to construct an electromagnet using various materials in order to attract an object, and this could open the safe or something.

We were given instructions in late October last year, allowing plenty of time to come up with decent ideas, buy the necessary materials, perhaps construct a few prototypes, and thoroughly test our final safe several times in order to make adjustments. We were also told that we were welcome to communicate with the physicists at Weizmann to help us come up with ideas.

Of course, we left it until the last two weeks to even finalise our design and start building the safe, let alone have time to test it. The night before the competition, a team member and myself were still busy gluing servos and switches into the safe and getting very frustrated by the fact that it never seemed to work properly. We would edit the code slightly, try again, and for some reason it would work and we would rejoice for a brief moment. Having changed absolutely nothing, the mechanism would then somehow decide not to work a second time. Deep down, I could sense the safe mocking us with its blank visage, revelling in both our failure and exhaustion. The squeak of the acrylic almost resembled that of a jeering laugh.

Four hours later at 5 am in the morning we were on the school minibus (‘on’ being a very loose term implying ‘presence’ – it would be better to say that mindless corpses could be found inside the minibus). Add another four hours to that, and we were inside the bustling city of London – lost, of course.

The main reason for today’s post is to share with everyone the interesting and innovative puzzles created by the other teams and their various physics principles. It would take far too long to go through them all, so I will picking out a few prominent safes and going through their solutions.

  1. Cup of Tea


First of all, a small fire is created by rubbing 9 V battery onto some steel wool. This happens because the battery induces a current in the thin metal wires which heat up to the point of burning; the steel fibres are very fine and have large surface area to volume ratio, letting them oxidise easily. This fire is used to light two wax candles, which are then held underneath a metal grating in order to heat two wires. The coiled wires contract because they are made of smart alloys which retain their original shape after being deformed and then heated. Doing this successfully allow a magnet to be obtained, which leads onto the latter part of the safe which is shown below.


Salt is poured into a beaker of water from above through a few small holes. Then by attaching a magnet to a small motor and putting another magnet into the water through a hole at the top, the water can be mixed by the two magnets attracting each other. This dissolves the salt, thereby allowing the sodium and chloride ions to conduct electricity, completing the circuit and outputting a code onto a display.

  1. The Grandfather Clock


A small wooden cross puzzle, comprised of two interlocking blocks, is solved by simply placing it on a flat surface and spinning it so that two wooden rods inside each block fly to the edges due to the virtual centrifugal force, thereby allowing the two parts to be taken apart. These two pieces are then used as keys to unlock the main part of the safe.

Various switches need to be hit in order which lie in the swing-paths of pendulums of different lengths. In order to increase the amplitude (the sideways swing) of each pendulum so that they are able to hit the switches with enough force, the centre of mass of a driver pendulum needs to be made as similar in length as possible to the target pendulum. When the driver is then swung, it will pick out the resonant frequency of the target pendulum because it has the same length. Hitting the switches in a correct order will allow crackers to move onto the final part.


Liquid is poured into a beaker, which displaces the air in the beaker through a tube as it has nowhere else to go. As the air is forced into a second beaker, the pressure increases and so the liquid is pushed up the final tube, and goes back into the first tube in a loop. Eventually, through a continuous process, all the liquid in the second beaker will be extracted through this process, and the safe is cracked. This system is known as Heron’s fountain. Yeah, this explanation is probably very confusing, as is everything else I’ve written thus far. If you desire greater clarity, please consult this YouTube video.

  1. The Theme Park Safe


First, the safe is lifted up off a base with a metal pole in the middle. Metal ball bearings can then be poured into concentric circles engraved into the base and the safe is then lowered back onto the pole, so that it can be easily rotated. The safe needs to be spun at high speed, causing some water in the middle of the safe to rise to the sides of a cylinder due to the virtual centrifugal force, obstructing lights from entering the cylinder. This unlocks the next part of the safe and turns a small filament bulb on inside the safe.


A piece of smart wire needs to be bent into the shape of a hook and then inserted through a hole so that it is adjacent to the hot filament bulb. The heat causes the smart wire to reform to its original shape, which is that of a hook. With the wire now hooked, the crackers can pull a small switch to crack the safe.

Sorry for the long, technical descriptions – I hope that you did not find the post too deterring. The safe puzzles may sound relatively simple written down, but given ten minutes and no indication of what to do whatsoever, the situation becomes slightly trickier…

Unfortunately our team did not make it to the second round in Israel, but it’s the taking part that counts, right?

Stay tuned and stay safe,





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