As The World Turns – Foucault’s Pendulum Explained

AS THE WORLD

TURNS

By Julie Caspersen

St. Andrew’s students can watch the world turn every day. Our Foucault Pendulum displays this motion, which has a renovated home at the entrance to the science and technology wing in McLaughlin Hall. The device is named after Léon Foucault, a French physicist who built one to prove that the Earth is not stationary but rotates. His pendulum had its public debut in February 1851 in Paris, attracting crowds invited “to see the earth spinning.” Fast forward 120 years, and SAC was gifted its very own, one of 13 in Canada. The donation came in 1971 following the death of a young student. Peter Dobbin ’73 attended from 1967 to 1969 and passed away at the age of 15. His parents, Cam ’45 and Dolly, chose to honour their son with a lasting educational gift that can be used as a teaching tool in physics classes. “It’s science in action. It’s not just science in theory,” enthuses Marke Jones, Head of Science. Having such a unique device in a school setting brings life to physics lessons. The pendulum is visible proof that the world rotates, something SAC students witness every day as they pass the pendulum and notice that its direction of swing changes. “The pendulum has several interesting physics concepts related to its motion. It also has practical applications in clocks or as a visible demonstration that proves the Earth spins on its axis,” says Jamie Inglis ’91, Associate Director, Student Life and Grade 11 physics teacher. “In physics, the goal is to make science tangible, make it relate to the students’ world.” During the recent renovation of McLaughlin Hall into a state-of-the-art science and technology wing that opened in February 2021, the pendulum also got an upgrade, thanks to Jamie, a self-described tinkerer. He updated the mechanism that controls the pendulum, which is governed by electromagnetism. “The control system was developed using concepts and technologies that we discuss in detail in the Grade 11 Physics course. The fact that we can study the motion of the pendulum as well as fully understand the mechanism that keeps it moving provides valuable learning opportunities for students,” he says. “Any opportunity that we have to link concepts with real-life tangible applications brings science to life and makes the subject more engaging for our students.” The pendulum’s motion is studied in the Grade 12 Physics unit on frames of reference and relative motion, and it is a constant fascination to students of all ages who, along with visitors to campus, can often be seen gathered at the glass casing. The SAC pendulum is a beautiful, scientific, and meditative installation – just one more thing that makes St. Andrew’s special. X

Air resistance would eventually stop the pendulum’s motion, so the pendulum needs to be given a little push to make up for the lost energy. An electromagnet pulls down on the steel ball underneath the bottom plate. This electromagnet is turned on when the pendulum’s swing makes the wire contact a ring at the top of the shaft. You can see this through the third-floor window if you go up the stairs. The magnet shuts off when the wire breaks contact with the ring. The extra pull from the electromagnet equals the drag of air resistance, and the pendulum keeps swinging forever — or until there is a power failure! Keeping the pendulum running properly is not quite as easy as it sounds because the ball must be released in precisely the right direction. An improper release causes the ball to move in an elliptical path and consequently does not receive the proper push from the electromagnet or, worse, can receive a sideways push that causes the ball to rotate at the wrong rate.

The plaque that hangs beside the pendulum honouring Peter Dobbin ’73

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FOUCAULT FACTS

A 7.5-metre (25-foot) wire supports a 10-kilogram (22-pound) stainless steel ball.

The pendulum’s 7.5-m length means it takes almost exactly one second to swing back and forth.

A pendulum at the equator shows no rotation.

The rotation of a pendulum’s swing at the North Pole is one rotation per day, which is 360 degrees in 24 hours, or 15° per hour.

SAC’s pendulum is about halfway between the North Pole and the equator (latitude 44° N) and rotates at 15 x sin(44°) = 10° per hour, so it takes about 36 hours, or one-and-a-half days to make a complete rotation.