# Physics 28 Building Arches

Lots of bridges have arches under them. The Romans used arches under their aqueducts for thousands of miles carrying water to their cities. What’s so special about arches?
Question: How does an arch work?
Materials:
Paper
Tape
Procedure:
Make two small diameter paper tubes

The two tubes should be a small diameter and about the same diameter.

Overlap the ends 2 cm and tape them together [You will need to flatten the ends.]

The two tubes only need 2 or 3 cm overlap but must be flattened for the overlapping part.

Bend this long tube into an arch but don’t fold it

The tape must extend over the paper joint on both sides.

Place one end of the arch against a book or other heavy thing
Hold the other end lightly to make it an arch
Press down on the center of the arch letting the end move if it wants to
Hold the end of the arch firmly in place
Press down on the center of the arch
If you can, suspend a cup from the center of the arch and fill it with weights
Observations:
Describe what happens to the end of the arch when you put pressure on the top:

Force applied to the top of an arch flows down each leg into the ground or other base under the arch.

Describe what happens when you hold the ends firmly and put pressure on the arch:
Conclusions:
Where does the pressure go when you press on the arch?
How does this make the arch a good way to build a bridge?
What must a builder do to use an arch safely?

When weight or force is applied to an arch, the base of the arch tends to spread. This tendency must be countered when the arch is used in construction.

If two arches are side by side, what happens to the forces acting on the bottom of one arch?

What I Found Out:
My arch tried to fold at the sides of the flattened part a little making the top look a little flat. When I pressed on the center of the arch the first time, the free end moved out letting the arch flatten.
The pressure I put on the arch went down both legs of the arch. The one leg was trapped against the box and wall so the force pushing that leg out was stopped by force from the wall. The free end had no force acting against it so the end moved outward.

Arches are more rounded than this paper version but this one shows how arches move forces so they can support a lot of weight.

When I held the second leg firmly in place, I supplied the force against the outward force. The arch held firmly.
I could not suspend weight from my arch but think the cup would hold a lot of weight. The force from the weight would go down the legs into the table and the wall and my hand.
Since an arch moves the force from weight off the bridge, the bridge can carry a lot of weight. Since the arch has an opening in the middle under it, other traffic or water can move freely below the bridge.
The problem with using an arch is the outward force acting on the legs. There has to be something strong pushing back to keep the arch in place.
Two arches side by side will do this for each other. The outward force from one leg pushes against the leg from the next arch which is pushing back as weight force moves onto it.