[Illustration: FIG. 11.]
28. The resolution or decomposition of one force into two forces each
greater than itself is capable of being illustrated in a variety of
ways, two of which will be here explained. In Fig. 11 an arrangement
for this purpose is shown. A piece of stout twine AB, able to support
from 20 lbs. to 30 lbs., is fastened at one end A to a fixed support,
and at the other end B to the eye of a wire-strainer. A wire-strainer
consists of an iron rod, with an eye at one end and a screw and a nut
at the other; it is used for tightening wires in wire fencing; and is
employed in this case for the purpose of stretching the cord. This
being done, I take a piece of ordinary sewing-thread, which is of
course weaker than the stout twine. I tie the thread to the middle of
the cord at C, catch the other end in my fingers, and pull; something
must break—something has broken: but what has broken? Not the slight
thread, it is still whole; it is the cord which has snapped. Now this
illustrates the point on which we have been dwelling. The force which I
transmitted along the thread was insufficient to break it; the thread
transferred the force to the cord, but under such circumstances that
the force was greatly magnified, and the consequence was that this
magnified force was able to break the cord before the original force
could break the thread. We can also see why it was necessary to stretch
the cord. In Fig. 10 the strains along the cords are greater when the
cords are attached at C and D than when they are attached at A and B;
that is to say, the more the cord is stretched towards a straight line,
the greater are the forces into which the applied force is resolved.