Learn more physics!
I would like to know how a gothic arch can stay up in place without falling. could you send me a discription or diagram of the angles and froces involved in the construction of a gothic arch. Thanks you very much.
Great site by the way!
- mark (age 13))
international school of paris, Paris France
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:
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.
(published on 10/22/2007)
Follow-Up #1: Gothic arch construction
You answered a question Mark (age 13) asked about how gothic arches stay up and not fall. However, my questions are: How are gothic arches constructed? How are the stones "put" in place so that they dont fall while constructing the arch? How are the side walls of the arch kept in place so that finally they come together at the top with the top center triangular stone? Thanks.
San Francisco, CA
Ah, this is one of the triumphs of the ancient Greek and Roman
engineers -- how to build a stone arch that relies on its compression
forces to stay up. In the process of building it of course not all the
stones are yet in place, and so something else must be there to keep it
up during construction.
The easiest solution is to build a wooden frame with supports for
the stones of the partially constructed arch. Once the arch is in
place, you can disassemble the frame and leave the arch free-standing.
Arches of course have lateral spreading forces from the rounded
top on the supporting columns. Either the columns must be thick enough
to handle the torque without collapsing, or secondary arches can be put
on the side of the supporting columns so they do not fall over
sideways. These can be constructed at the same time as the main arch
with wooden supports.
(republished on 07/12/06)
Follow-up on this answer.