25 March


Checking in

Checking in

Natasha is taking part in a physics contest and has won a trip to Israel to participate in the finals. She left today along with the other members of the team from the school.

The Shalheveth Freier Physics Tournament is organized by the Weizmann Institute of Science and is based on having students compete in teams to build the most effective physics-based impregnable safe. The idea is that the locking mechanism for the safe needs to be based on basic physics, but that it be at once both simple and yet obscure, so that the safe cannot easily be opened by those who don't know the trick.

In addition working on the design and construction, she ended up being in charge of packing the safe, which sat on our dining room table for a week as she planned how to pack it.

A previous entry, for example, involved the use of tuning forks that had to generate specific notes, and a computer inside the safe that computed Fourier transforms to identify the selected notes.

I can't explain the principle behind the St George's safe at this time, just in case one of the competing teams stumbles on this web page. A detailed example of a prior winning entry follows.

Safe locking mechanism from De-Shalit High School, Rehovot – Winners of the 2001 tournament.
This Safe locking mechanism is based on items found within a student’s typical pencil
1. Radiation emitted by electric charge: When a charged particle is accelerated
– an electromagnetic radiation is emitted. If this acceleration is periodic, the
radiation has the same frequency. In a typical calculator electrons are accelerated
at the frequency of its clock all through its electric circuits, giving rise to RF
2. Change of polarization by reflection: The polarization of light is defined as
the direction of its electric field vector. One may always define two orthogonal
polarization directions in the plane perpendicular to the light direction. When light
is reflected from a surface, the two polarization components typically are reflected
with different efficiency. For some angles the reflected light is predominantly
made of one component only, hence it becomes polarized.
3. LCD display: LCD displays, such as that of a calculator, are made of a pixel
matrix. Each pixel is made of liquid crystal, which changes the polarization
according to the strength of the applied electric field. To display the desired
monochromatic image the display is illuminated by polarized light and an electric
field is applied to the appropriate pixels. The polarization plane of the light
reflected from these pixels turns. The light is then reflected back through a
polarizer transparent only to the unperturbed component. Hence, pixels subject to
an electric field remain dark, thus generating the desired image.
To open the safe one should follow three steps using objects found in a typical highschool
student pencil box (all are provided with – but outside of – the safe).
1. Close the electric circuit by a pencil. For this purpose one has to sharpen both
ends of the pencil and place it in the gap between both contacts (cf. figure). Its
central part is made of carbon, which is known to be a good conductor.
2. Using the calculator one may generate EM waves by hitting some keys. Placing
the calculator outside the transparent door next to the receiver, the received
radiation is amplified and the generated current opens an electromagnetic switch.
3. The polarizer of the calculator within the safe has been removed. Hence the
number on its display cannot be seen directly. This number is the code for the
mechanical lock attached to the door of the safe. To read the display one should
use the plastic ruler. Placing it (outside the transparent door) at the appropriate
angle, the reflected image of the display is polarized and the number can readily be

By Gregory Dudek at | Leave a comment |    
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