*** 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 butresses.

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 butresses.

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 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.

*** BASIC PROCEDURE AND EXPLANATION ***

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 hand 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 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 buliding house 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 butresses) in. Let go and it stays up! Why? Again, because the buttresses push inwards.

Explain that they're called flying butresses. 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 butresses we could use thick walls and put weights on top. Try it. Why does it work? Extra weight adds extra force in direction required. Thus 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!

*** OTHER THINGS TO TALK ABOUT ***

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...

*** SCIENCE BACKGROUND FOR DEMONSTRATORS ***

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