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INTRODUCTION The state of reduced acceleration of gravity has become an important subject of research in the advanced study of human be- havior and tolerance. In aircraft, because of the limitations in speed and altitude, such a condition occurs very infrequently dur- ing certain maneuvers, such as pushovers and zooming, and then for relatively short periods of time only. But with rocket pro- pulsion the situation is quite different. After burnout the rocket cruises entirely unsupported, and in this coasting state the missile and its passengers are in a condition of zero-G. Hence, weight- lessness is the psycho-physiological steady-state in rocket flight. This pertains to the relatively short durations of IRBM and ICBM missions, as well as to the longer exposures during orbital trips and travel into outer space. Although the aviation physiologist has been long acquainted with the principles of powered flight and the effects of increased acceleration, he was not aware of the phenomenon of reduced ac- celeration and weight until very recently. Moreover, it was not always sufficiently clear which physical quantities were involved in the entire spectrum of these biodynamic processes, and which designations, terms, and symbols are adequate for the descrip- tion of their effects. The consequence was the improper use of terms, such as, "gravity-free", "zero-gravity", "null-gravity", "subgravity", and "weightlessness". However, from the stand- point of a proper terminology it is absolutely necessary that the conditions of reduced gravity are clearly defined, and that these definitions are in complete agreement with the concepts of physics and mathematics. In bioastronautics it is convenient to express acceleration as a multiple of the standard acceleration go, and any force as a multiple of the standard weight Wo of the body upon which a force is acting. This is equivalent to setting up an auxiliary system of practical units, in which the UNIT ACCELERATION: 1 g (in magnitude) = 32. 2 ft/sec2; and the UNIT FORCE: 1 W (in magnitude)3 32. 2 m lb. 1
It should be noted that the imit of acceleration in this system is al- ways a true constant, while the unit of force may differ for bodies of differing masses. Since we define -â ⢠G* (1) io Wo the quantity G is dimensionless, being the ratio of two accelera- tions or of two forces. This unit indicates how much force or acceleration is present in a given dynamic situation. Since the actual force on a body --- due to the acceleration --- is the weight of the body, G also indicates weight. It should be emphasized that the small letter "g" has been quite rigidly established by the physi- cist as the symbol for a specific physical quantity, namely, as the unit for the acceleration of the earth's gravity -32.2 ft/sec^, normally. Any other usage constitutes a distortion of meaning that may create unnecessary confusion. In this age of technology it is mandatory that we fully comply in our denotations with the symbols representing mathematical and physical quantities. In a recent paper;** an attempt has been made to define weight and weightlessness parameters which correspond to the physical conditions and physiological experiences of man in aircraft and rocket maneuvers. These concepts may also be of benefit for the preparation of experiments involving motion simulators and zero-G devices. In all cases of freely moving bodies, the force of inertia compensates the gravitational force at any point of their trajectory, thereby creating the so-called "agravic" or "gravity-free" state. However, this is not necessarily so because the body is always under the influence of gravitation, either of that of the earth or of any other celestial body (e. g. , the artificial earth satellites gravi- tate around our planet; the early"lunar probes", around the sun). In this state the components of a body or a system are identically affected by the prevailing gravity- inertia relationship; that is, they are not appressed in the direction of the original gravitational vertical. Consequently, this state has been designated as "ap- pressionless"; but it is more accurately defined as the "zero-G" *WO = standard weight of a body go = standard acceleration of gravity **Ritter, O. L. and S. J. Gerathewohl: The concept of weight and stress in human flight. A. U. School of Aviation Medicine, USAF, Randolph AFB, Texas, 58-154, January 1959.
or "null-G" condition, since the resultant force exerted on a body due to gravity and inertia is zero. Weightlessness, on the other hand, has been widely used to describe the psycho-physiological experience of an individual in an unappressed state. For example, a body immersed in water although supported by the surrounding medium and under normal gravitational conditions is apparently weightless and not ap- pressed to a scale placed under it. The true "agravic" and the "zero-G" condition, on the other hand, occur only if there is no gravitational force acting, or when the acceleration of gravity is fully counteracted by inertia, respectively. According to the phy- sical characeristics of the agravic and zero-G condition; i. e. , the gravitational-inertial relationship extended to at least the molecu- lar level of the masses involved, their technological and biological effects generally should be the same wherever and whenever they occur; that is, within or outside of any gravitational field, and during free fall as well as during free ascent. However, the in- homogeneity of the field forces involved may give rise to inter- molecular forces, which may account for a non-identity of the biological effects of the agravic and the zero-G condition. This can be decided only by future experimentation.
