Grade 11 physics and the Mystery of the Swaying Cross

SooToday.com received the following note this morning from Andrew Hambleton, a local high school student who applied some lessons from his Grade 11 physics class to the mystery of Sault Ste. Marie's swaying Cross on the Hill.

To review our earlier coverage of this story, please click on the links that appear at the bottom of Andrew's letter.

For other reaction (including some insights from retired journalist John Campbell), please check the News Response forum in our Editorials section.

************************** Hello, my name is Andrew Hambleton.

I am a high school student in Sault Ste. Marie. While browsing SooToday.com I came upon the stories of the swaying of the cross, and with all the professional opinions expressed by engineers and whatnot, I decided to formulate my own opinion on the happenings of that day.

I am surprised that this idea was not already expressed by the professionals, so maybe I'm way off here, but it makes a lot more sense to me than the ideas of "underground caves" or "earthquakes," which SooToday.com reported.

While reading the story I immediately thought that perhaps this phenomenon was a direct result of the the cross's structural resonant frequency.

Basically, wave-like patterns of wind being thrust on a structure at just the right frequency can cause a structure to waiver due to repeated gusts multiplying each other's effects.

To simplify, it is like pushing a toddler on a swing.

If you time the pushes correctly, you can generate a lot of lift or sway and a good height, with relatively no effort.

The first push moves the swing a little, but the second push adds to the momentum of the first giving more sway, while the third push adds to the effect even, and so on.

Eventually significant movement can be noticed.

An incidence of such events occurring is found in the case of the Tacoma Narrows bridge.

On November 7, 1940, at approximately 11 a.m., the first Tacoma Narrows suspension bridge collapsed due to wind-induced vibrations.

Situated on the Tacoma Narrows in Puget Sound, near the city of Tacoma, Washington, the bridge had only been open for traffic a few months.

What witnesses described as light winds brought down the bridge because the timing of the gusts was just good enough to sway the bridge, each gust adding to the last(constructive interference).

I believe that this is possible in the cross's situation because with resonant frequency, it does not matter that the cross is strong or took on high forces of wind in its existance, ("It has survived some pretty good wind forces in its time. It was there when the Edmund Fitzgerald went down and it is on a highly exposed area," he said.)

This is becasue it is not the force of the wind that matters in a situation like this, it is the timing of the wind gusts acting on the structure.

The case of the Tacoma Narrows bridge gives an extreme example of how this effect caused by noticable winds brought down the bridge.

In the case of the cross, witnesses say it was a calm day.

I feel that this fact does not refute my theory, as the height of the cross is probably great enough that there were currents of wind acting on the top portion that were not experienced at ground level.

(Witnesses say that the middle section of the cross stood still, suggesting that said currents of wind acted even above this point)

Some information, pictures and video taken from the tacoma case can be found at:

http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacoma.html

I'm not sure my theory can help at all in the search for the true explanation of the phenomanon, but it at least gives light to a new and (to my knowledge) unexplored idea.

If you do not think this is possible, I would be interested in a professional's opinions on the matter as I am only a student, taking all of my knowlege of the subject from a Grade 11 physics course, and a bit of my own research.

*********************** Full SooToday.com coverage of this story

Miracle on St. George's Hill? - Did the big cross move? Engineer can't explain swaying-cross reports Grade 11 physics and the Mystery of the Swaying Cross Soil stability problems are nothing new on St. George's Hill