Glossary
### Couple

- A pair of equal, parallel and opposite forces which
are not in the same straight line. A couple provides a pure torque or
turning effect. The value of the couple is given by the joint turning
effects of the forces about the mid-point between them, which is the same as
the product of their distance apart and one of the forces. Sometimes it can
be convenient to introduce the idea of a couple to an arrangement of forces
to make clearer what is happening. As a simple example, consider the
following:

Suppose a force P acts down on the end of a cantilever of
length L. Apply equal and opposite vertical forces P1 upwards and P2 downwards
at the support. (On their own these would have no effect.) Then P and P1
together form a couple (P x L) tending to turn the beam ,
and the remaining force P2 tends to push the beam downwards. Thus, the load P
has both rotating and translating effects on the beam.

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