Page:The Kinematics of Machinery.djvu/54

 32 another way than before. So soon as the force $$Q$$ begins to act it calls forth in the interior of the wheel, the shaft and the supports, internal molecular forces, opposite in direction and exactly equal to it. The action of the forces therefore, considered by themselves, is here exactly the same as, or exactly corresponding to, their action in the case of the satellite. There exists however the difference that there, while here the action of an external force becomes at once the   of the opposite action of a molecular force.



In actual machines we do not employ absolutely rigid material, for no such material exists; we use however only those materials which when of suitable dimensions alter their form under the action of external forces very little, so little that the corresponding variations from the original form may be neglected, The choice of suitable dimensions and forms is the work of the machine designer. If we disregard the very small variations of form which actually occur, it appears that the solution of the problem by the machine exists, and also that it  from that occurring in nature:

Whilst in the first system, which we may call, the external measurable mechanical forces are opposed by similar external forces, in the second, the   system, there are opposed to all external forces others concealed in the interior of the bodies forming the system, and appearing there,—and acting in