Composition of Parallel Forces

The resultant of more than two parallel forces is compounded in a manner similar to the composition of non-parallel forces. Two forces are taken in turn and their resultant is then combined with one of the remaining forces.

A system of forces equal inmagnitude and opposite in direction, acting at two points, not in the line of force, is called a couple and tends to produce rotation, in place of translation. Therefore, when a body possesses both translation and rotat ion we may assume the translation to be produced by a force and the rotation by a couple. The perpendicular distance between the lines of action of the two equal forces is called the arm of the couple. The produet of one of the two equal forces and the arm is called the moment of the couple and is the measure of the power of the couple to produce rotation.

As any number of forces can be compounded into a single force, a given force can be resolved into two or any number of forces which would produce the same effect. These mechanical principles operate in precisely the same mechanical manner from the playing of a five-finger pattern to a Beethoven Sonata. We can appreciate them as the foundation for any investigation of piano technique. Consequently, they will be referred to frequently in the later analyses of physiological movement.

The principle of the lever points out the fallacy of locating the muscles that move a part in the part moved. The direction relationships explain the great speed obtained in arm, hand, and finger movements with relatively little muscular contraction, since most muscles are levers of the third class acting in the maximum speed direction. The couple gives us a mechanical basis for the important forearm rotation touch.
These points and their manifold ramifications are taken up in detial in the subsequent module on Touch-forms.