Science Puzzle
Why Doesn’t the Leaning Tower Fall Over?
The Leaning Tower of Pisa is a stone bell tower about 56 metres tall, weighing around 14,500 tonnes. It began to tilt while it was still being built in the 1100s, because the soft ground under one side sank. Today it leans by about 4 degrees, yet it has stood for more than 800 years.
Why doesn't the tower topple over?
The Answer
Its centre of gravity is still above its base. As long as that's true, gravity keeps pulling the tower down onto its foundations instead of round and over.
Every object behaves as if all its weight acts at one point, the centre of gravity. Draw a line straight down from that point. If the line lands inside the base, the object is stable: tip it a little and its own weight rocks it back. If the line falls outside the edge of the base, the weight starts turning the object over the edge, and it topples. For a tall, narrow tower, that edge isn't far away.
The lean was getting worse. By 1990 it had reached about 5.5 degrees and the tower was closed to visitors. Between 1993 and 2001, engineers carefully drew out about 38 cubic metres of soil from under the high side, letting that side settle a little. The lean came back to just under 4 degrees, moving the centre of gravity safely back towards the middle of the base. The tower has even straightened by a few more centimetres since.
You can feel the same rule in your own body. Stand with your heels against a wall and try to touch your toes without moving your feet. You can't: your centre of gravity has to move forward of your feet, and the wall won't let your bottom move back to balance it.
The principle: Stability and the centre of gravity. An object stays upright as long as a vertical line through its centre of gravity falls inside its base, and it topples once that line passes outside the edge.