# Centre of Gravity/Mass

When a tall chimney falls, it usually breaks in two at some point along its length. Why doesn’t it fall in one piece? Where will the break occur? Will the chimney bend towards or away from the ground after the break? If the chimney does not break, something even stranger may occur: the base of the chimney may hop into the air during the fall. How can it do this seemingly against gravity?

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phinds
Gold Member
2019 Award
The characteristics of a chimney falling will be very dependent on its construction. A solid concrete one can act way different than a brick one (I'm assuming you're talking about factory-sized ones, not house-sized).

I'm not sure what you mean about the "hop". Can you be more descriptive about a specific instance?

I am talking about factory sized chimneys indeed, and I am wondering how BOTH solid concrete chimney and brick chimney would act when they fall. When the base of the chimney hops into the air during free fall I am thinking that wind resistance and gravitational accelration causing large internal stress result in the chimney breaking before it even makes an impact on the ground. Thanks

phinds
Gold Member
2019 Award
I am talking about factory sized chimneys indeed, and I am wondering how BOTH solid concrete chimney and brick chimney would act when they fall. When the base of the chimney hops into the air during free fall I am thinking that wind resistance and gravitational accelration causing large internal stress result in the chimney breaking before it even makes an impact on the ground. Thanks
I still have no idea what you are talking about on this "hop" thing.

do you have some videos that show some examples?

sophiecentaur
Gold Member
The 'hop' must be something to do with a (transverse) wave that is set up as the chimney hits the ground progressively along its length.
When comic actors fall over backwards, their feet always go up in the air, too.

K^2
When a tall chimney falls, it usually breaks in two at some point along its length. Why doesn’t it fall in one piece? Where will the break occur? Will the chimney bend towards or away from the ground after the break? If the chimney does not break, something even stranger may occur: the base of the chimney may hop into the air during the fall. How can it do this seemingly against gravity?
Chimneys are built to withstand great compressive forces along their length, but not that strong of the side-to-side forces. When it falls over, to move at constant angular velocity along its entire length, the end of the chimney must experience much greater acceleration than the middle, and the bottom is almost stationary. This puts the stress on the chimney as if the middle is being pulled down, while the top and bottom are being pulled up. So no wonder that the chimney snaps more often than not.

The hop part is trickier, I have a thought on this, but I'm going to need to write it up and see if what I think happens actually makes any sense.

K^2
Ok, this took a while, so I hope it helps clear it up a little.

Keeping in mind that the chimney does break in half, lets model it as two rigid halves connected by a joint. The joint will be allowed to rotate freely to simulate breaking point. Keep in mind that this will greatly exaggerate the result. So the first image attached is an animation showing the simulated tower collapsing. Tendency to snap in half is absolutely clear. Second image is the plot of the force required to support the tower at the base. The initial spike is from the model settling in. From there, the force stays fairly level for a while, and then decreases as the tower leans and collapses. Finally, right before the tower crashes into the ground, something interesting happens. The force goes negative. That means that the bottom of the tower tries to "hop" into the air, and my constraint, requiring that the tower stays planted, is not letting it.

It's not perfectly clear to me why this happens. I suspect, the top half pressing down on the far end of the bottom half causes this, but I'm not sure how the top half ends up moving in such a way as to cause that. Either way, this is purely an outcome of tower segments being fairly rigid and interacting with each other.

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Thank you so much K^2 your knowledge is much appreciated.