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Public summary: 

See how simple shapes can fit together to hold up a large structure, like a cathedral!

How can we use arches to hold up a cathedral?
Useful information
Kit List: 

Base - three sections connected with hinges x1
Foundation blocks with metal connecting pins x4
Large arch connected with elastic cord x1
Small arches (buttresses) connected with elastic cord x2
Thin walls x2
Thick walls x2
Weights x2

Frequency of use: 

*** OVERVIEW ***

Investigating how the ideas used to make arch bridges can be used to build cathedrals and other buildings. Looking at the foundations needed for arches and flying buttresses.

Possible Activities:
Building a single large arch without walls but with foundations.
Building a single large arch with thick walls.
Building a single large arch with thin walls and weights.
Building a single large arch with thin walls and flying buttresses.

Other things to talk about:
Resolving forces in different directions.
Examples in real cathedrals (see photos).
Foundation depths and strengths.

Tips for demonstrating:

Best demonstrated if the children have already looked at the arch bridge experiment, as then you can link the two experiments nicely.

Be careful with the weights as they do pose a finger-trap and bruising hazard - best to get them to hold the wooden arches, well away from where the weights will drop, whilst you position the weights in the correct places. It is very difficult to get the weights to balance, so I often just get the kids to press vertically down on top of the block with their index finger. This achieves the same effect, but you should make clear that this is the same as putting a big weight on top.


1. Starting off

Explain that it's a cathedral. We're trying to build one. What are cathedrals built from? Stone. How to get a stone roof, window or door to stay up? Show them a photo - why doesn't the roof fall in? Ask if they've ever been in a cathedral and what shapes did they see. Arches!

Explain that the model is a cross section of a cathedral. This is actually quite difficult - young children are not used to models and cut-away views so you'll have to wave your hands and show them what would go where.

2. Large arch falling in

Now you can do the actual experiment. Start with the large arch and place it straight on the base (without supports). Ask the children to hold the two ends of the large arch and ask them what happens when you let go of the ends. Let go of the ends and it falls down.

Why does this happen? Normally children say something along the lines of "nothing is holding it up" but you can go a bit further than this. Try to get across the idea that you need something to push the ends of the arch inwards, as follows.

Repeat it but rather than getting them to hold the ends of the arch, ask them to keep the arch up by pushing only with one finger. Most children then try different directions until they realise they have to push the ends of the arch inwards. It can be difficult to get this working with two children but let them persevere and give them clues if necessary.

When they've done it, ask which way they're pushing. If they're old enough you could talk about forces but be very careful! Bear in mind they might not have heard the word "force" before and even if they have they might not know what it means - it can prove a bit of a distraction and just bore them.

3. Large arch with foundations

Once you've done that, put the foundation bricks in (match up the numbers (1 & 2) to ensure a good fit) and ask what will happen when you let go now. Don't mention "foundation" yet.

The arch stays up. Why? Why didn't it fall down? Previously we had to use our fingers to push inwards to stop the arch falling down, and that's what the bricks do.

What are the bricks called? Foundations. Talk about house construction, danger of building houses on uneven or muddy/sandy ground.

4. Arch with walls and buttresses

What is the problem with this cathedral so far? No walls, ceiling very low!

Try putting the arch on top of the thin walls. What will happen when you let go? It falls down. Why? Get them to explain, using the finger method like before, that we need something to push the arch inwards. If we don't have anything pushing in then the arch falls down.

Now put the smaller arches (flying buttresses) in between the outer foundations (numbered 3 & 4) and the thin walls. Let go and it stays up! Why? Again, because the buttresses push inwards.

Explain that they're called flying buttresses. Have they seen any? Show photos - where are the flying buttresses in the photos. What are they holding up?

5. Thick walls and weights

This next bit is quite subtle and you might not want to try it with young children.

Instead of flying buttresses we could use thick walls and put weights on top. Try it. Why does it work? Extra weight adds extra force in direction required. This is very subtle - children need to understand forces and have some idea about how forces in different directions add up. Don't try it unless you think they'll get it!

This is quite tricky to do, so make sure you practice it before trying with the children. First place the thick walls at the locations marked on the base, then rest the arch (matching up the A and sides) on the marked part of the top of the thick walls. The arch won't stay up at this stage so keep holding it up. Next place the thin walls (upside down and oriented so that the top surface is flat) on top of the thick ones and add the weights to the top of the thin walls. The weights need to be as close to the centre as is possible without them falling off, the structure should now stand up on its own.


Applications with bridges.
How old buildings were made
Cement as a glue for bricks.
How modern buildings are made.
Look at the buildings around you.
How does your roof at home stay up? Look in the attic - timber beams...


Using the science behind arch bridges to make buildings.
Resolving forces - how adding weights/using thicker walls, stops the roof from caving in.

Risk Assessment
Date risk assesment last checked: 
Tue, 04/02/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Tue, 04/02/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 

Various small wooden arches are built to explain flying buttresses and pinnacles:

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Elasticated wooden blocks Slight finger trap between the elasticated wooden blocks All 3 2 6 The bungee holding the blocks together isn't too tight, but be aware of the problem and ask children to move fingers if they are in danger of being trapped. Call a first aider in the case of an injury. 2 2 4
Brass weights Dropping one of the brass weights on a finger. All 3 2 6 Try not to leave or let kids lean on the floor by the bridge. Brass weights may easily be replaced by getting kids to press down with a single finger. Call first aider in the event of injury. 2 2 4
Wooden blocks/base Getting splinters from blocks or the base. All 3 3 9 Ensure all blocks and the base are well sanded. If not, sand them or don't use them. Call first aider in the event of an accident. Aim to keep children calm around experiment. 2 2 4
Wooden blocks on floor Wooden blocks may be trip hazards. All 4 3 12 Keep blocks in same area and out of public walking areas. Call first aider in event of injury. 2 3 6
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Publicity photo: 
Experiment photos: