The Leaning Tower of Pisa has been leaning for over 800 years and, despite earthquakes, hurricanes and wars, it is still standing.
However, a Team of engineers is constantly monitoring its inclination and people in Pisa are ready to take action to save the Tower should its inclination become critical.
But what is a critical inclination and why the Leaning Tower of Pisa does not fall?

Here are the answers...

It is all about geometry

Every object has a mass and it has something called center of gravity (or center of mass). The center of gravity is that point inside the object where all the mass should be concentrated if we want to approximate the object with a point and keep its inertial properties. Holding an object by its center of mass keeps it in perfect equilibrium as gravity acts with the same torque in any direction around that point.
While this concept comes handy when you are in school studying physics (or if you want to impress your friends balancing a knife on your finger) it has little or no use in everyday life.

An object stands until the line drawn from its center of gravity to ground falls within its base.

leaning tower pisa center of gravity example

When talking about the Leaning Tower of Pisa, the concept of center of gravity comes to be very important.
Any object that stands (typically on its base) will keep standing (or falling back onto its base) until the line drawn from its center of gravity to the ground falls within its base (see image on the right).

The mass of the Leaning Tower is about 14500 metric tons and, given its geometry (thin at the top and thick at the bottom), its center of mass sits somewhere lower than the top of its 4th floor.

leaning tower pisa galileo gravityToday, the Tower leans so much that a vertical line drawn from its outer edge (on the 7th floor) would meet the ground 4 meters far from the base.

Somewhen around the year 1600, Galileo Galilei took advantage of the unique geometry of the Tower to conduct experiments on gravity by throwing objects down from its leaning side.
He successfully demonstrated that the mass of an object has no influence on the speed he gathers while falling towards the ground.

In 1600 the Tower was leaning much less than today and its offset to the ground was surely not 4 meters. However, it was enough for Galileo to carry on his experiments and write an important page of history and science.

Galileo used the Tower to prove that mass of falling objects has no influence on their speed.

The Tower is falling

When in 1990 the Tower reached the inclination of 5°, engineers warned that its center of gravity was coming dangerously close to the outer edge of the base... in other words, the Tower was coming close to topple over.
A massive rescue operation was launched and the Tower was closed for "straightening works". It took 30 million EUR and an entire decade to execute the works that brought back the Tower to an acceptable inclination (more on this in the article on how the Tower was stabilized).

The Tower has always been slowly increasing its inclination since the end of its construction in 1372. We can say that the Tower has always been falling.
This dangerous behavior was foreseen by its constructors and who built the Tower was clever enough to trick its geometry in a banana shape. This way its center of gravity falls a little more behind the geometric center of the base and it allows the Tower to lean a bit more than it could if it was perfectly straight (like any other tower on the planet).

Why it still stands

In the end, the Tower is not falling because its center of gravity has been carefully kept within its base.
At first, it was by cleverly modifying the design (by giving the Tower a banana shape) and later, when the inclination became too much, the center of gravity has been brought back up with straightening works.

The Leaning Tower doesn't fall because its center of gravity is carefully kept within its base.

It is very unlikely that the Tower will fall over in the foreseeable future since it is closely monitored and the works executed two decades ago improved greatly its stability.

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