Leaning Tower

The Leaning Tower of Pisa

The Leaning Tower is in Pisa in North West Italy. It was built over a period of about 200 years and was started in 1173.


The Leaning Tower is an amazing structure. It is a very beautiful piece of architecture but it is also very unusual because it leans over at an angle which makes it look like it’s going to fall over!

In fact it was in great danger of falling over in the 1990’s and scientists, engineers and architects argued over the best way to save it from disaster.

It became so dangerous that people were banned from going near it in case it collapsed.

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On the right is a model of the Leaning Tower built by pupils at Dunn Street Primary School in Jarrow.

The challenge was to build the tower then simulate how Professor John Burland (see below) stopped it from falling over and brought it back to a safe angle so that people could start visiting again.

First the class did a series of investigations related to strength of structures and balance. They looked at ways of finding out how strong a structure is and why an object will overbalance and fall. They also found out about the Forces in action in a structure and what these forces can do eg. cause stretching, bending, compression and breaking or ‘shear’.

The tower was tested in a box of sand to simulate the soil conditions under the tower and around the foundations.

The three main investigations on the tower were as follows:

1. Using a cable in tension to stop the tower leaning any further. The cable (a string) was ‘looped’ around the middle of the tower then gradually pulled until it just started to move the tower back to a smaller angle of lean.

2. Heavy weights (jars of marbles and a lump of lead) were added to the base of the tower on the side opposite to the direction of lean. This stopped the tower leaning any further and actually decreased the angle of lean even more. The tension was seen to reduce in the cable and it became slack.

3. Soil (sand in this case) was removed by scooping it out from underneath the tower on the side opposite the direction of lean and this caused the tower to lean less and move back to a safer position.

The three investigations promoted lots of discussion about the forces in action and what these forces might do which could affect the tower.

eg. If too much lead weight was placed on the base of the tower it might actually crush and break the stone from which the tower was built, including the foundations under the ground. The tower might simply have broken in pieces.


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The Force of Gravity was very important in the discussion of forces acting on the structure.

Research by the pupils revealed that a very important scientist tried an amazing experiment using the tower!

He was an Italian named Galileo Galilei (1564-1642). He studied at the University of Pisa and it is said that he climbed to the top of the tower and released two cannon balls of different weight to see how quickly they would fall to the ground.

He said that both cannon balls would travel at the same speed and so they would reach the ground at the same time! He was correct!

The most accurate way of doing this experiment is in a vacuum (in the absence of air or any gas) because air resistance can slow the descent!

The experiment was repeated on the moon by the astronaut Neil Armstrong. He didn’t use cannon balls, instead he used a hammer and a feather! Lots of people thought that the hammer would hit the moon’s surface first!

They both landed at the same time just as Galileo had predicted.


The Moon’s gravity pulling down on the hammer and feather gives them both the same acceleration. They pick up speed at the same rate and so they hit the ground at the same time.

The Moon’s gravity is about ‘one sixth’ of the gravity on Earth because the moon has far less mass than the Earth so objects fall more slowly on the Moon.

Galileo became known as the ‘Father of Science’ because he made people realize that the things we see going on all around us can be explained by science eg. the phases of the moon or the motion of falling objects. He was a ‘philosopher’ or great thinker.

The famous person who saved the tower was John Burland. He is the ‘Professor of Soil Mechanics’ at Imperial College in London. He is a Scientist and Engineer. He studied the problem and came up with a brilliant way of stopping the tower from tilting any further and even made it begin to tilt back in the other direction a little bit to make sure it was no longer in danger of falling.

Another problem that the professor had was to try and put right the mistakes which had been made in the past by other scientists and engineers which almost caused the tower to collapse instead of making it safe.

The tower was so near to falling over that even the wind blowing against the tower from a particular direction might have provided enough force to push it over!

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