At the beginning of this book we discussed the so-called inertial forces and we distinguished several types of these forces: gravity, inertia and, as a special case of the latter, centrifugal force. At this point, we must concern ourselves in somewhat more detail with their nature.
It is the nature of these forces that they do not act only upon individual points of the surface of the object like other mechanical forces, but that they act simultaneously on all points even its internal ones. Since this special characteristic feature is common to all inertial forces, it is, therefore, entirely immaterial as far as a practical effect is concerned what type of inertial force is involved. It will always affect an object in the same fashion, as the force of gravity, and we will likewise feel it in every case as the well-known "weighty feeling," regardless of whether the force is gravity, inertia, centrifugal force or even the result of several of these forces. As a result of this complete uniformity of effect, it is possible that different types of inertial forces can mutually strengthen or weaken or also completely cancel each other.
We are already familiar with an example of the occurrence of a mutual strengthening of inertial forces when studying the ascent of space rockets. In this case, the force of gravity is increased due to the resulting inertia as long as there is thrust, something that makes itself felt for all practical purposes like a temporary increase of the force of gravity (Figure 22).
Figure 56. Carousel, accordance to Oberth. This equipment and that shown in Figure 57 are both designed to produce artificially the condition of an increased force of gravity for the purpose of carrying-out physiological experiments.
Key: 1. Counterbalancing weight; 2. Vehicle; 3. Pneumatic cushioning; 4. B Lateral arm; D. Tracks.
However, even under normal terrestrial conditions, the state of an increased force of gravity and even for any desired duration can be produced, when the centrifugal force is used for this purpose. Technical applications include, for example, different types of centrifuges. Their principle could be applied even on a large scale using a carrousel built especially for this purpose (Figure 56) or, better yet, in specially built giant centrifuges (Figures 57 and 58). At an appropriately high rate of rotation, a very significant multiplication of the gravitational effect would be achievable in this fashion.
Figure 57. Giant centrifuge according to the author's recommendation. This equipment and that shown in Figure 56 are both designed to produce artificially the condition of an increased force of gravity for the purpose of carrying out physiological experiments.
Key: 1. Beam with a slight clearance of motion; 2. Ball bearing; 3. Maintenance platform; 4. Tubular pole made from sheet iron or iron lattice tower; 5. Gondola for the experiments; 6. Drive motor; 7. Backup brake; 8. Braking occurs normally by the motor using energy recovery; 9. Concrete base; 10. Drive shaft.
Figure 58. The giant centrifuge in operation.
Key: 1. Gravity; 2. Centrifugal force.
On the other hand, a longer lasting decrease or cancellation of gravity (that is, generating a continuous weightless state) is not possible under terrestrial conditions, because to emphasize this once again the force of gravity cannot be eliminated in any other way whatsoever than through the opposition of another inertial force of the same magnitude. Therefore, an object can be prevented by supports from falling (i.e., responding to the force of gravity). Its weight, however, cannot be cancelled, a point proven by the continual presence of its pressure on the support. Any experiment to remove the influence of the force of gravity from an object, for instance, by some change of its material structure, would, no doubt, be condemned to failure for all times.
On the Earth's surface neither a correspondingly strong different force of gravity is available nor can centrifugal forces be generated in an object in such a way that it is transposed into an observable weightless state as a result of their effect.
Figure 59. The interplay of forces on a free falling object.
Key: 1. Inertia (activated by the acceleration due to gravity); 2. Free falling object; 3. Weight (as a result of the Earth's attraction); 4. Acceleration due to gravity of 9.81 m/sec2 (caused by the weight).
It is, however, possible on the Earth if only for a short duration to offset the force of gravity through the third inertial force, the force of inertia. Every day, we can experience this type of occurrence of weightlessness on ourselves or observe it on other objects, namely in the free fall state. That an object falls means nothing more than that it is moved towards the center of the Earth by its weight, and, more specifically, at an acceleration (of 9.81 m/sec2, a value familiar to us) that is exactly so large that the force of inertia activated in the object as a result exactly cancels the object's weight (Figure 59), because if a part of this weight still remained, then it would result in a corresponding increase of the acceleration and consequently of the inertia (opposing gravity in this case).
In the free fall or during a jump, we are weightless according to this reasoning. The sensation that we experience during the fall or jump is that of weightlessness; the behavior we observe in an object during free fall would be the same in a weightless state generated in another way. Since, however, falling can only last moments if it is not
supposed to lead to destruction (the longest times are experienced during parachute jumping, ski jumping, etc.), the occurrence of the weightless state on Earth is possible for only a very short time. Nevertheless, Oberth was successful in conducting very interesting experiments in this manner, from which conclusions can be made about the behavior of various objects and about the course of natural phenomena in the weight-free state.
Completely different, however, are the conditions during space travel. Not only can free fall last for days and weeks during space travel. It would also be possible to remove permanently the effect of gravity from an object: more specifically and as already stated in the introductory chapter by using the action of inertial forces produced by free orbital motion, in particular, of the centrifugal force. As has been previously stated, the space station makes use of this. An orbiting station is in the state of complete freedom from gravity lasting indefinitely ("a stable state of suspension").