Foucault Pendulum and the Coriolis Effect (page 2)

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Author: Alex Jacobsen


Your Foucault pendulum likely moved in a small ellipse as it went back and forth. This is okay. After several minutes, you should have noticed the Pendulum moving a small distance away from its starting line. If you’re in the Northern Hemisphere, it should have moved in the clockwise direction. If you’re in the Southern Hemisphere, it should have moved counterclockwise. Regardless, it will likely not have moved very far, especially if you live close to the equator. At 30 degrees latitude your pendulum will precess about 2 degrees in 10 minutes. It will be more if you live farther from the equator, and less if you are closer.


            So how can we explain what we witnessed? We already know that the earth spins on an axis. We also know Newton’s First Law, which states that things will not change their behavior unless acted upon by an external force. In the case of your pendulum, nothing is actually forcing it to change the direction it is swinging in, so it will continue to swing in the same direction as the Earth turns underneath it. The pendulum’s bob doesn’t change its orientation—everything else around it does! We’re essentially witnessing our earth creating the Coriolis effect: an object looks like it’s moving because it’s viewed in a rotating reference frame. If you were at the North or South Pole, you would see the pendulum’s direction rotate all the way around (360 degrees) in one day. But since it’s unlikely that you’re actually conducting your experiment in either of these locations,, the pendulum’s direction will rotate a little more slowly (and at the equator, the pendulum actually wouldn’t change its direction at all)..

            Now think about why you were asked to make your pendulum swing in such a small arc. We did this to keep other behavior from dominating the movement of the string—this tends to happen when the string bends too far relative to its length. The Foucault Pendulum’s effect is extremely difficult to see, and anything that interferes with it will make it practically undetectable. Most Foucault Pendulum exhibits in science museums are around 40 feet long with a 50-lb weight! This is also the reason you need to very gently let the pendulum swing, because small changes can cause the pendulum to begin travelling in an ellipse, making it difficult to see the pendulum’s precession. You need a very long pendulum to make those effects go away and allow you to see the true action of the earth’s spin.

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