World keeps turning

Oct 192005
Authors: Lee Newville

This month marks the fifth anniversary of three members of the physics department setting out to prove the rotation of the Earth. Their project still swings in the Engineering building.

It's called a Foucault pendulum, and it works by creating an optical illusion. Although things are different here in Colorado, and more complicated, if you watched the pendulum swing back and forth for a long period of time at the North Pole, the ball would appear to slowly shift clockwise. However, it is actually the Earth beneath the pendulum that moves.

In other words, the building, attached to the Earth, rotates 360 degrees every day. The pendulum would swing in the same plane all day due to a force called inertia, creating the optical illusion. However, this is only achieved at the North and South poles.

"It turns out that when you look at it at CSU it goes around in some 39 hours," said physics professor William Fairbank. "So there's not a simple explanation of the Earth turning underneath it."

CSU's pendulum actually does rotate along with the Earth underneath it. This is explained by a phenomenon known as the Coriolis effect, which describes the slight deflection of objects observed from a rotating frame of reference. Because of this, the pendulum does not behave as it would if it were located on the top of the North Pole. It is the Coriolis effect, however, that proves the rotation of the Earth.

French physicist Leon Foucault crafted the idea in the 1800s. In 1951, he built the first formal Foucault pendulum in the Pantheon in Paris. It was the first dynamical proof of the Earth's rotation.

The pendulum in the Engineering building was dedicated to then Professor Emeritus of Physics Lawrence Hadley in October of 2000. It was constructed during the renovation of the Engineering and Physics building and replaced an older pendulum in the original building, built in 1957.

"I'm honored. It came as a complete surprise to me," Hadley said about the dedication before his death in 2001.

Associate Professor of Physics Richard Eykholt said the dedication was fitting because Hadley had previously made his own amateur pendulum.

"When he was a faculty member here, he had built a pendulum that hung from the top floor to the basement," Eykholt said. "It was a very simple thing he put a lot of effort into designing."

The new pendulum "was a very fancy version of something he had built many, many years ago."

Unlike many more expensive pendulums in museums, CSU's is powered by an electric circuit at the top, which keeps the ball moving against the force of air resistance. The steel ball attached to the 25-foot cable weighs 265 pounds, which helps keep the pendulum swinging by reducing friction over a polished stone compass rose on the floor.

"Lawrence was a longtime faculty member here," said David Krueger, chair of the physics department. "He was very much involved with doing demonstrations to illustrate physics principles, so this was a natural thing to dedicate to him."

The pendulum was conceived, constructed and designed by Dave Warner, Bob Adame and Jay Jablonski.

To see live footage of the pendulum swinging visit

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