## Moving Linked Rods

Many mechanical components rely on linkages of rods that form a mechanism
that can move in certain ways, partly constrained and partly flexible.
Such mechanisms are often used to transfer power to drive machine parts.
The bars that drive the wheels on a steam locomotive are a common
example, as are the arms that control dentists' drills. In flat
linkages, the bars are joined by bolts or slip joints that constrain the
motion to two dimensions. In other linkages, some joints rotate around
an axis, thus permitting motion in three dimensions.
A flat linkage of four bars in the form of a quadrilateral will flex in
two dimensions, but once the lengths of the bars are fixed, only certain
motions are possible. One use of such a linkage is to transform circular
to oscillating motion. Suppose A is the center of a motor that turns rod
AB of a quadrilateral linkage ABCD in a circular motion. Vertex D is
also fixed, so as B moves in a circle around point A, C moves along a
path determined by the lengths of the four rods. In one particular
application, the fixed base AD is 5 feet and the rotating arm AB is 1
foot long. What motions are possible for point C for various lengths of
rods BC and CD?

Suppose a fifth rod is added, making a flexible pentagon with vertices
ABCDE. A and E are located at the centers of motors that turn rods AB
and ED in circular motions. If AE is 5 feet, and AB and ED are 1 foot
each, what happens to point C as AB and ED turn at different rates?

*Comment:* Workplace examples such as linkage problems are quite
common. They involve simple mechanisms that turn, slide, and rotate to
make things go. Anyone who has tried to clear a paper jam from the
inside of a Xerox machine has seen multiple examples of mechanisms at
work. Anyone who tries to fix such a machine needs to understand how
geometry serves the cause of making things move. Problems of this type
exercise geometric thinking in powerful ways. They cry out for physical
models, which are easy to build. Many variations are possible, and all
have good physical applications. The mathematics involves applications
of coordinate geometry as well as trigonometry and simple algebra. Much
of the analysis can be carried out either by geometry, analytic geometry,
algebra, or spreadsheets.

Excerpted from "Mathematics for Work and Life" by Susan L. Forman and
Lynn Arthur Steen (in *Seventy-Five Years of Progress: Prospects for
School Mathematics,* NCTM, 1995).

*To contribute or correct items, please e-mail
information to: *`extend@stolaf.edu`* or click
here.*

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*Last Update: 12/29/98*