Classical Mechanics rests, among other things, upon the foundation of Isaac Newton’s equations of motion. In its simpler form, Newton’s Third Laws, for example, states that, “For every action, there is an equal and opposite reaction.” This might be thought of as a conservation law, in the sense that a net external force on a body will result in an equal and opposite, reactive force by the same body. This leads ultimately to other fundamental laws such as the conservation of energy or conservation of momentum.
There is, however, unwritten Assumptions in Newton’s Third Law. Such assumptions are clearly a fundamental aspect of the law, but all too often they are not stated explicitly with the law. This omission results in the law’s misinterpretation, or a faulty assumption as to the realm of applicability of the law.
These assumptions can be phrased in the following ways: 1) Newton’s Third Law is only applicable to a mass without dimensions (i.e. a “point mass”). This assumption is extant despite the fact that virtually by definition a “point mass” cannot exist in nature -- for a body to have mass inherently implies that the body also has dimensions. Therefore, for a force to act upon the body, it will in the general case act upon one portion of that body and not upon the entire body all at once, or act upon different parts of the body with different intensities of force.
A second, related assumption is that the force acts upon the entire body simultaneously. If we apply a force, for example, to the end of a rod, before the rod can react to this force (in accordance with Newton’s Third Law), the “action” of the force must make it’s way along the length of the rod, and then return to where the force is being applied (and thus be in a position to react to the external force). Only in this way can the full mass of the rod communicate its presence to the impinging force.
It is a fundamental reality of physics that the rod -- or any massive object -- acts as if all of its mass were concentrated at its center of gravity. Accordingly, the application of a force, must communicate its presence and magnitude to the center of gravity of the rod. Inasmuch as the speed at which this communication is transmitted along the length of a typical rod is on the order the speed of sound in the medium of the rod, any “transmission time” of the information (regarding application of the external force) is quite short. In the case of a metal, for example, this transmission might be on the order of one ten-thousandth of a second (0.0001 seconds).
The “transmission time” viewpoint allows us view the second assumption of Newton’s Third Law as an assumption of absolute time (or absolute simultaneity), if the action is at a distance. This assumption continues despite the reality that the “reaction” of a body to an externally applied force can not be simultaneous with the initial application (the “action”) of the external force. This profound restriction on Newton’s Law brings time into the equation -- a restriction which becomes the critical factor.
It should be noted that this concept of a non-simultaneous “reaction time” to an applied force (whatever the nature of that force is considered to be) is applicable not only in mechanics, but also in electromagnetism and other fields of physics. A sudden surge (an “action”) of current along a conductor, for example, will also result in an equal and opposite reaction -- but again not simultaneously. Likewise a rotating shaft surrounded by permanent magnets arranged so as to impel the rotation will encounter an equal and opposite magnetic force which will brake the rotation -- after a time delay, or what might be called the “Critical Action Time” (CAT)! 
The point to be emphasized here is that the laws of mechanics are applicable in the electromagnetic realm and vice versa, and that mechanics, electromagnetism, and other areas of physics are not separate or unrelated. The same form of differential equations often apply in different fields, along with the form of their solutions.
There is yet a third assumption of Newton’s Law that is often glossed over. This is the idea that the body upon which an external force acts, is a rigid body. It’s rather as if when you push a large lump of jello, you may get a equal and opposite reaction by the jello on your finger, but the jello itself may not accelerate as a body whose dimensions have not been altered by the imposition of the external force. This latter assumption has profound implications in situations substantially more interesting that pushing against jello (or as exemplified in the old adage, “Don’t push the river.”).
Consider, for example, an artillery shell (generally assumed to be a rigid body) impacting upon a sheet of armor plate (also assumed to be rigid). As the artillery shell first strikes the armor plate, the leading edge of the artillery shell (call it the first wave of atoms in the makeup of the artillery shell) is forced backwards by the Coulomb repulsion between electrons in the outer shells of the atoms in both the artillery shell and the armor plate. This first wave, however, quickly finds itself repulsed by the atoms in the artillery shell immediately behind the leading edge (i.e. the “second wave”). (One can also think of this as “being caught between the rock and the hard place.”) The process continues with succeeding waves of atoms in the artillery shell, until the leading portion of the shell is oscillating against the armor shell -- effectively hammering its way into the armor plate. Meanwhile, the trailing edge of the artillery shell proceeds to deaccelerate in a non-oscillatory fashion. In effect, under the extreme deacceleration of the artillery shell upon impacting the armor plate, the artillery shell proves itself not to be a rigid body -- a fact which has been experimentally observed! 
The fundamental key to all of this is that something not generally appreciated about Newton’s Third Law is the fact that the law fails at extreme accelerations (or in this case, deaccelerations), and in addition, when the body has sufficient size that the time delay -- the “transmission time” which communicates the imposition of a force to the center of mass (or “center of inductance” in an electrical circuit). This point has been addressed by Davis, Stine, and others [1, 2, 3], who realized that the non-simultaneity and possibly non-rigid nature of reality comes into play under conditions of extreme accelerations.
More significantly, it becomes ever more clear that one can initiate an action upon a body, and then detach before the equal and opposite reaction can fully manifest itself.
If a force is applied to an object and there is a time delay (the “Critical Action Time”) before the object can fully resist the application of the force, then the force (which has already initiated the action upon the object) can retract and avoid the object’s reaction. This is similar to what we do when we hammer a nail. Just as the hammer is about to make impact, we loosen our grip on the hammer such that the shock the nail and the hammer receive at impact is not transmitted into our hand. This is particularly true when we’re dealing with a sledge hammer. (Or at least after the first, body-shattering blow.)
This also relates to the popular notion in the movies that someone can hit someone else’s head with their head, and while the recipient falls over unconscious, the initiator of the action escapes Scot free. How can that be? Because the initiator also retracts his head quite quickly, and therefore avoids the results of the impact -- what in physics would be called the impulse. The recipient has been sent reeling back, his head well hit, while the initiator is walking away as if nothing happened.
Numerous, additional examples can be found at The Fifth Element.
This principle can be used in advanced propulsions systems, in what we would refer to as Inertial Propulsion Systems.
In mainstream forms of propulsion, there is the essential requirement of something called reaction mass. Basically, this means that in order to propel anything, something else must be pushed against, or reaction mass expelled. Rowing a canoe is pushing against the water in order to move in the opposite direction. In a rocket, small masses are violently thrown in the opposite direction of the rocket’s intended travel, but at very high speeds (thus the mass times velocity, the momentum, is the same for both the rocket and the reaction mass being expelled.) [There is also the very strange, theoretical means of travel via Quantum Wormholes, but this requires “ghost radiation” or negative energy, which is even more esoteric and wildly speculative than propulsion without reaction mass!]
In Inertial Propulsion, the trick is to do something very quickly and avoid the need for reaction mass. Think of it as going into a bank to rob it, but being so quick that you can get out of the bank with the money before the bank can react. In effect, you can get away with anything, if you exit the system sufficiently quickly. (Of course, if you’re not faster than the monitoring cameras, the ‘system’ you will need to exit might include the country.)
This process also shows up in the case of yanking a table cloth from a table without the dishes crashing to the floor*. The Experiment denoting the distinction between inertial mass and gravitational Mass is also a demonstration of the basis for Inertial Propulsion.
[*Last Thanksgiving Day, there was a world disaster, when someone (probably a brother-in-law) attempted to yank the table cloth from the fully loaded dining room table. It was the downfall of Turkey, the overthrow of Greece, and the destruction of China.]
Inertial Propulsion is therefore the use of Mach’s Principle and Connective Physics to accomplish one’s need to move it! The key is to do a series of impacts (impulses) with a sufficiently high, tuned frequency that one never has to pay the reaction mass price.
Note, however, that Inertial Propulsion is not necessarily the same as Levitation, inasmuch as the latter often requires a gravitational object to push against.
But as we progress merrily along, it may be comforting to know that the inertia of which we’ve so often complained, may become a wonderfully useful resource of getting around.
Even going backwards...
As well as forward:
 Davis, W. O., Stine, G.H., Victory, E.L., and Korff, S.A., “Some Aspects of Certain Transient Mechanical Systems”, Presentation to the American Physical Society, New York University, April 23, 1962.
 Stine, G. Harry, “Detesters, Phasers and Dean Drives”, Analog Science Fiction / Science Fact Magazine, June, 1976.
 Davis, William O., “The Fourth Law of Motion”, Analog Science Fiction/Science Fact Magazine, May, 1962.
2003© Copyright Dan Sewell Ward, All Rights Reserved