Arches - how strong are they?
The hypothesis that larger arches are able to withstand a heavier load is proven to be true to a certain extent only. As the curve of the arch increases, so does the surface tension on the upper surface of the plywood, leading to breakage.
Arches have been used in construction since the 2nd millennium BC by the Mesopotamians. Ancient arch bridges built by the Romans are still structurally sound and some of them are still standing today.
The science fair project can be repeated by varying the type of wood used.
The experiment can be repeated by varying the width of the plywood used.
Arch - http://en.wikipedia.org/wiki/Arch
How bridges work? - http://science.howstuffworks.com/bridge6.htm
Sawing should be performed by an adult wearing proper safety gear.