Bill Aycock on sat 13 mar 99
Gavin- There is a very real distinction between the curve known as a
"catenary" and the curve known as a "parabola". The math for a parabola is
extremely simple, but the plotting of the curve, to lay out an arch, is not
as simple, for most non-math people.
On the other hand- the math for a Catenary is complex, while the drawing of
the curve is "dirt-simple".
Simply put- the curve for the Catenary describes a path in which there is
no bending force - a chain or other limp, linear object serves well to
describe it. Even a rope, however, has enough bending strength to make a
poor model.
The simple way to describe a good catenary arch is - upside down. Lean a
sheet of plywood at a slight angle to the vertical- put two nails in the
sheet, the width of the desired arch, at its widest, and above the bottom
edge by the height you want. Get a peice of Ball chain (thats the stuff
used on electrical light pullchains- available on spools at do-it-yourself
building suppliers). Fasten one end of the chain to one of the nails- drape
the chain so it touches the bottom edge and the other nail. A simple way to
mark the plywood is to spray (Lightly !!) with a contrasting paint.
Voila-- thats a CATENARY - no math- no calculation. (Note- it is usual to
use the inside dimensions (width and heigth) when laying out the arch.)
Good luck- Bill
At 03:32 PM 03/12/1999 EST, you wrote:
>----------------------------Original message----------------------------
>Clayarters:
>While we're on the subject will members of the list make a distinction
>between a Catenary arch derived from hanging a chain/rope from two points
>that mark the span to a point which marks the rise and a Parabola? From a
>mathematical perspective are they one and the same or are they merely very
>closely related? TIA.
>
>Rafael
>
>
>-----Original Message-----
>From: Gavin Stairs
-
Bill Aycock --- Persimmon Hill
Woodville, Alabama, US 35776
(in the N.E. corner of the State)
W4BSG -- Grid EM64vr
baycock@HiWAAY.net
| |
|
