ABSTRACT
The application of some external force or action system to a body will produce a reaction system which either maintains the state of rest or (by Newton's laws) will cause acceleration to take place and to continue until changed circumstances restore equilibrium. Thus the application of load to a bridge produces internal stresses and reactions from the foundations of the abutments which together provide a loaded system at rest. Alternatively the increase of thrust from an aircraft engine causes acceleration of the aircraft, until the increased drag of air resistance and the other combined aerodynamic forces restore equilibrium at some uniform (but increased) velocity. In both cases, the action and reaction systems (which might be very complex) can each be compounded to give six force vectors which will be equal in magnitude and opposite in direction.
