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Make a clean break

Terry O'Dea describes how students used physics to crack a safe

Looking for a novel way to raise your students' understanding of the A-level physics of mechanical oscillators (for example, Edexcel A2 physics, unit 4, Waves and our Universe)?

The physics department at King Solomon High School in Ilford came up with an ingenious idea for the topic of simple harmonic motion, which emerged during their participation in a safe-breaking competition at the Weizmann Institute in Israel earlier this year. In the competition, the team of five students had to make a device which would open a standard electromechanical lock on a "safe" (a wooden box with transparent door and desirable booty displayed inside). Their design is easily made, and it consolidates students' understanding of pendulum-and-spring simple harmonic motion.

Opening the safe is a two-stage challenge. The first section begins with a steel mass-loaded spring hung vertically from an oscillator, driven by a signal generator. The spring has a resonant frequency which can be found by adjusting the frequency of the generator. At the resonant frequency, the maximum amplitude causes the load to get near to the lever switch, but not near enough to activate it. (The switch is there to trigger the release of the pendulum mass, held by an electromagnet in the second section.) The key to opening the safe lies in finding the right resonant frequency of the spring, by adjusting the mass and the frequency of the generator, to give sufficient amplitude for the load to reach the switch. The simple harmonic motion equation T = 29 C KM highlights the relationship between the mass and the resonant frequency 1T (K is the spring stiffness constant, M the load mass).

What is needed is more mass. A disc magnet resting on the end of a ruler is inserted into a hole in the side of the box, enabling the magnet to attach to the load, increasing the resonant frequency of the spring. With the right number of magnets, the new resonant frequency - found by trial and error (or calculated by some students) - provides the necessary amplitude to hit the switch, which in turn causes the short-circuiting of the electromagnet circuit in section two and releases the simple pendulum.

The section two task is to cause the pendulum to swing with sufficient amplitude to activate the final switch that opens the door. Again, an understanding of the way coupled pendulums, of equal length, transfer energy between each other will, with a little thinking, solve this problem.

A coupling length of string threaded through the side of the box, a convenient clamp, a stand, string and bob can be arranged so as to create the appropriate resonating system between the two pendulums. The internal pendulum contacts the switch and the prize can be retrieved. Such a piece of kit will provide many thought-provoking and useful experiences for future physics students.

l The students in the team, pictured with teacher Jo Sheehy (second left) and their safe, were (from left) Sam Taub, Josh Greene, Craig Summer, Gilbert Dwek, Garry Blair.

* The Weizmann Institute employs 2,500 postgraduates in pure and applied research across many science disciplines.

For physics links: For the annual physics tournament: Terry O'Dea is a science consultant at the London borough of Bexley


* A strong box

* Hinges and a Perspex door

* Electromagnetic lock mechanism

* Oscillating driver, spring and steel mass

* Signal generator

* Steel pendulum bob

* String and a ruler

* Ceramic magnets

* Microswitch wood screws

* Connecting wire

* Electromagnet

* Clamp and stand.

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