If you happen to be around when they are knocking down a chimney (AKA a smokestack), you might notice it breaking in mid-air as it falls. This observation is characteristic of all falling tall structures – especially chimneys. It has been termed, “The Falling Chimney Paradox“.
Here is how this phenomenon unfolded before a multitude in 1980.
The falling chimneys of Kempton Hardwick
One chilly morning in the summer of 1980, some 10,000 people gathered in Kempton Hardwick, Bedford, England. They had come to witness what many considered to be a significant moment in Britain’s final chapter of the Industrial revolution. An event that was personal to most Britons of the area was imminent.
It was the simultaneous demolition of 18 chimneys of the London Brick Company at Coronation Brickworks. These chimneys had played a significant role in shaping the towns and cities of England. At their peak operation, these chimneys could produce 130 million bricks a year – that’s enough to build over 15,000 houses!
The public had received the news of the demolition differently; former workers were saddened to see the landmark demolished. For conservatives, this would effectively mark the end of Britain’s Industrial sovereignty, for environmentalists, this was a tremendous step toward a better, greener Earth and for the Guinness World of Records, this was a new record.
As the explosives went off, and the chimneys came tumbling down, one sight was common in the midst of different implications imposed by the demolition:
Why do the chimneys break as they fall?
Indeed, almost all of the chimneys broke as they fell. Here is the video for the event. Begin from minute 7:30 for a better view.
What this article is about
The interesting thing about this observation is this; it is not a coincidence. When a tall structure (such as a chimney) is tipped over, it almost always breaks in mid-air as it falls. The real question here is why this happens.
This inspires the objective of this article; to find out why tall structures break as they fall, and to explain that in a simple, non-technical way – like to a fourth grader.
As it turns out, physics can provide an explanation for this observation. Even better, physics can predict where along the height of the chimney a break is most likely to happen. Cool huh!
Tall structures act weird when they fall
This weirdness was termed ‘the Free-fall paradox’ by physics experimenter Richard Sutton in the 60s while studying the dynamics of falling bodies. He observed that, if a tall object (such as a meter rule) is made to fall by tipping it over so that it traces an ark, the free end of it falls faster than a freely falling body under gravity.
In other words, if you dropped a pivoted meter ruler and coin from the same level, with the ruler inclined at around 35° to the horizontal, then the endpoint of the rotating ruler will hit the ground sooner than the coin. Implying that the endpoint of the ruler accelerated faster than a free-falling coin, and arrived at the ground first. This is counter-intuitive as one would naively assume that a body falling freely under gravity should at least fall faster than a hinged one.
So this is the weirdness: bodies that fall while following an arc, actually fall faster than freely falling ones.
Falling stick experiment
A more fascinating version of this experiment involves a meter stick, a small steel ball, and a beaker. The stick is hinged at one end, while the other end is inclined by making use of a prop. A small ball is placed on a tee, at the free end of the hinged stick, and the beaker is fixed at some distance from the free end. The stick is made to fall by quickly yanking the prop from under it. Here is a 2 min YouTube video demonstrating this!
As you can see, the ball lands inside the cup. This observation is counter-intuitive. Because had the ball and the stick fell at the same rate, then the ball should have stuck by the stick and not separated from it. This only suggests that the falling end of the stick fell faster than the freely falling ball. Detaching itself from the ball.
Making sense of rotational motion
As anyone who has ever ridden a Playground Roundabout or a Merry go Round can attest, you travel much faster when you are away from the center of rotation than when you are closer to the center. In fact, if you don’t want to move1 while playing on a roundabout then you should stay at the center.
Or just look at a rotating ceiling fan. You’d realize that you can “see” the center clear and sharp while the blades appear blurry. This would imply that the blades are moving much faster than the center of the rotating ceiling fan.
Point is, the closer you are to the axis of rotation of a rotating body, and the less distance you cover in a given time. Another way of expressing this statement is, the closer you are to the center, the less fast you travel. And if the rotating body accelerates, different parts of the body would accelerate differently.
So here is the main point. Different positions2 of a rotating body have different linear velocities and acceleration. The further a point is from the center the bigger the linear velocity and linear acceleration3.
Breaking due to stresses along the length of the chimney.
Well, since the falling chimney is effectively moving in a rotational motion, then different parts of the falling chimney have different linear velocities and accelerations. In fact, a point farther away from the bottom of the chimney, where the axis of rotation is located, the bigger the linear velocity and acceleration. Consequently, the taller the chimney the bigger the differences in accelerations and velocities between the top of the chimney and the bottom.
So while the chimney does fall as one rigid body, different parts of the chimney experience different accelerations as they fall down.
These different accelerations introduce stresses along the length of the chimney which, at some point, the mortar and concrete making up the chimney cannot withhold, and the chimney breaks in mid-air.
This is, of course, a very course, account for the dynamics of a falling chimney. I avoided going deep on certain concepts pertaining to rotational motion because this piece is intended for the common reader. However, if you’d like a more rigorous account of the dynamics of falling structures, I would suggest reading this piece published by the University of South California Beaufort, and this article, “Faster than ‘g’” published on the Harvard website.
I hope this article was useful, maybe next time you can predict what will happen when civil engineers are about to wreak havoc on a standing chimney or a tall structure! Share if you found this useful.
See you on the next piece!
- Here we are talking of linear movement. That movement involves changing linear position with time as opposed to rotation.
- Relative to the axis of rotation.
- The corresponding angular quantities are the same for all positions of the rotating body. That is, angular displacement, velocity and acceleration are the same for all points on the rotating body.
