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The Falling Chimney*


Equipment Needed:
Falling chimney
Sad ball(un-bouncy ball)

WARNINGS!!!
When the chimney falls, it falls relatively fast. Make sure that all hands and fingers are out of the way of the chimney.

What to Do:
The falling chimney consists of two boards of uniform length (see "Modification" below) hinged together. One board has foam strips along its bottom and the other has both a cup and a golf tee attached. The foam stripped board is placed foam down on a flat surface and the upper 'cupped' board is raised via the hinge and a stick of set length. A small ball is placed on the golf tee and then the stick is quickly removed. Set the falling chimney up as seen below. There should be a special mark where the stick should be put. Ask the students what they think will happen to the ball when the stick is pulled. Pull the stick toward yourself and see what happens. The ball should fall into the cup every time. This may require practice. If something is set unevenly or the chimney slips on the surface you are working on, the ball may just miss the cup. If it does, try it again. When the ball makes it into the cup, ask the students why they think it happened. A popular answer is that the ball hit the ground and bounced in. Show the students that the ball does not bounce.

The Falling Chimney Diagram

Why It Is:
The figure shows a 'time lapsed picture' of the falling chimney demo. We can assume that gravity acts on the center of mass of an object. If you drop two objects, their centers of mass (com) will fall at the same rate (approximately, see note). For a sphere, the center of mass is just the center of the sphere. For a board, the com is slightly shifted forward from the middle of the board. In the falling chimney demo we assume, the com of the board falls at the same rate as the ball. Because the board is stiff and hinged, the parts of the board farther from the hinge than the com fall faster. Since the com of the board starts lower than the ball, and they fall at the same rate, the board (and thus the cup) will hit the table before the ball. So, even though the cup starts above the ball, it will hit the table first, and the ball can easily land in it.

As you can well imagine, there is a little bit of a trick to getting the ball to fall directly into the cup. It is reasonable from the above explanation of center of mass that the cup will hit the ground first, but the reason the ball goes into the cup is because of some manipulation on our part. It was mentioned before that there is a special place where the dowel must be located before you pull it out. One way to find this special place is to draw a little line on the bottom board where the center of the cup is when the cup is not at an incline. Incline the board until you can line the golf tee up with this mark. This way, when the ball falls straight down, it will fall straight down into the cup. Practice a little until you get it just right and mark where the dowel should be.

What to Say:
This demonstration goes well with the ball demonstrations where you talk about things falling at equal rates. Once the students have this concept down, show them the falling chimney. This will get them thinking. Everything falls at the same rate (in the absence of air resistance), so why is the ball going into the cup?

Now you can fine-tune your explanation of everything falling at the same rate by telling them that the center of mass of everything falls at the same rate (in the absence of air resistance or other constraint). Let the students try the demonstration for themselves until they are convinced that this is what is happening.

Note:
As was mentioned, the center of mass of the ball and the center of mass of the board, don't actually fall at equal rates. It turns out that due to the constraint on the board, the ball falls faster than the board. For older audiences, ask them why they think this is. If they have already seen the inclined plane demo, they may have a guess. The board is not only falling, it is also rotating.

Modification:
The speed with which the board falls is dependent on how close the com of the board is to the hinge; the closer it is the faster the board will fall. One way of moving the com closer to the hinge is to change the shape of the board. The figure below shows one simple way of doing just that.

Modified Falling Chimney

* Please See Also:
George Barnes, "Some Physics of a Falling Lamp Post", The Physics Teacher, Vol. 30, Feb 1992.

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