Mark -
The thing that holds any kind of arch together is gravity. Gravity is a force that pulls everything on the earth straight down. The key to a gothic arch is that it has a (basically) triangular piece at the top:
Since gravity is pulling the top piece straight down, this means that all the forces produced by gravity on that top piece have to add up to being straight down. But it can only push against the flat edge of the bricks next to it. So what actually ends up happening is that it pushes the brick on its left a little bit to the left and a little bit down, and it pushes the brick on its right a little bit to the right and a little bit down. The "to the right part" and the "to the left part" cancel each other out, so that the total force being produced by gravity on the triangle is just straight down. The force on each brick to the side is then transfered much the same way to each of the rest of the bricks the rest of the way down. If this doesn’t exactly make sense, you can think of it in terms of some of the basic laws of physics. One of these laws is that "every force has an equal and opposite force" (for something that’s standing still). This means that the force of the top brick pushing down (gravity) is equal to the force of the bricks below it holding it up. Also, this means that the force that the top brick applies to the brick on its left is equal to the force that it applies to the brick on its right. The reason that the bricks to the left and right don’t just slide off to either side is because there’s a lot of friction (or mortar) between them and the bricks below them, so they won’t budge to either side.
Hope this is what you’re looking for.
-Tamara
(published on 10/22/2007)
(published on 10/22/2007)