This youtube video explains clearly the different types of bridges and forces. There are two forces, tension and compression. Different types of materials are best for withstanding each forces. There are a variety of different types of bridges, and each one corresponds to the forces differently. Depending on the shape of the bridge, certain materials are the best. It also displayed a diagram for where tension and compression are on a truss bridge. From this video I learned the physics in bridge building. I think this will be a good foundation for building my own bridge.