SYSTEM OF WEIGHTS

495

Fig 12.

A Couple — When two equal and opposite forces act at different
points on a lever a turning force, called a couple, is introduced which
can only be neutralised by another couple tending to produce rotation
in the opposite direction.

Examples.—Letting go the handle of the winch when the weight
is on, or letting go the capstan bars when the pawls are up; the crank-
shaft of a reciprocating steam engine; breasting a ship round the corner
of a dock wall, as in Figure 13, where Fis the fulcrum, P the power and W

W

Fig 13.

the weight. In the event of the resistance of the water on the starboard
bow and on port quarter being equal to the turning force of P, then
the couple would be neutralised and the turning of the vessel arrested.

CENTRE OF GRAVITY OF A SYSTEM OF WEIGHTS.
Example —(i) A wooden batten A B, graduated in feet, is supported
at A. A weight of 20 Ibs. is suspended at B 8 feet from A, The
moment about A is 8 ft. X20 lbs.—160 foot-lbs. (Figure 14).