(Premier Login aceputt) Forum Owner Posted Apr 29, 2009 11:10 AM
Dear sammy,
Your thought about "forces always act in equal and opposite directions" meaning something "static" is typical "naive physics" figuring-out. What Newton's Third Law really means is that when two inanimate objects collide (or one collides against another), there is only ONE FORCE acting on both objects. The effect of this force on the two objects is not, however, "static" or "equal" unless the objects are equal in shape, mass, density, etc., and collide with centers of mass head-on. Usually, a big truck smashes into a small car. Such a head-on collision has ONE FORCE acting on both objects equally in opposite directions, but the effects are not the same. The big truck experiences a bump and slows down a little while still going forward, but the small car gets SMASHED FLAT and is sent rocketing backwards! Nothing "static" about that!
Although physicists speak about "pairs" of equal and opposite "forces", they might as well speak only about a single "force" that acts in two directions equally on two objects during a collision. There is no functional difference in these two conceptualizations, once the forces are accepted as "equal and opposite". Here is the usual explanation:
"Newton's third law: All forces in the universe occur in equal but oppositely directed pairs. There are no isolated forces; for every external force that acts on an object there is a force of equal magnitude but opposite direction which acts back on the object which exerted that external force. In the case of internal forces, a force on one part of a system will be countered by a reaction force on another part of the system so that an isolated system cannot by any means exert a net force on the system as a whole. A system cannot "bootstrap" itself into motion with purely internal forces - to achieve a net force and an acceleration, it must interact with an object external to itself."
"Without specifying the nature or origin of the forces on the two masses, Newton's 3rd law states that if they arise from the two masses themselves, they must be equal in magnitude but opposite in direction so that no net force arises from purely internal forces."
"Newton's third law is one of the fundamental symmetry principles of the universe. Since we have no examples of it being violated in nature, it is a useful tool for analyzing situations with are somewhat counter-intuitive. For example, when a small truck collides head-on with a large truck, your intuition might tell you that the force on the small truck is larger. Not so!"
The DEEP MYSTERY here is just the symmetrical simplicity of the universe. No inanimate object carries around any (net) INTERNAL force that can act either on itself or on another object. That "rejects" the medievalist "impulse" concept of forces being "put in" objects. In this sense, the universe does not allow changing a mass by "putting speed into it". That's not how "force" operates. But there is a MUCH DEEPER MYSTERY, because according to Special Relativity objects near the speed of light become heavier and time slower. But that's "relative" to something else. In itself, an object at ANY speed cannot have anything "put into it" like a force that can cause something on another object later. Masses are not kangaroos with secret pouches hiding little kangaroos of force. "No net force can be generated within the system [internally] since all internal forces occur in opposing pairs. The acceleration of the center of mass is zero."
The whole concept of "force" is pretty fuzzy, despite what physicists may claim about the ontological reality of "force". This aspect of the science is highlighted by contrasting the concept of "force" with that of "energy". How do these two formulae relate to each other?
F = m*a
E = m*c^2
Well, m = F/a and m = E/c^2, so F/a = E/c^2.
Rearranging:
F = E*a/c^2
Where'd the "mass" go? Where's the "object" now? This formula says that a "force" is just a change of energy over a space in time. Okey dokey. There is no "mass" apart from its energy. "The mass of an object is a fundamental property of the object; a numerical measure of its inertia; a fundamental measure of the amount of matter in the object." Okey dokey, what is "inertia"? No one seems to know the answer to that one.
The interesting point that all objects either remain at rest or remain at constant velocity until operated upon by an external force means that in the absence of force, ALL OBJECTS have ZERO acceleration at their center. That sounds akin to "inertia", but why is either situation the case in our universe? Why do objects internally never have anything bu ZERO acceleration at the center, and why does mass have inertia?
Ultimately, this game peters out in the defining of the units for E and F by a committee of scientists all speaking the same language and no other language (mathematics / symbolism). What the hell is a "Newton" and what is it's relation to an "Erg"? Who knows, since it doesn't advance the understanding of the concepts or explain reality more clearly. Units typically codify mathematical relations, but mathematical relations are usually simple "observations" expressed in symbols in a simplistic correlation (equal to, greater than, etc.), and are not "propositions that reveal reality's inner secret." F = m*a is really just an observation, and not something Newton derived by experimental fact gathering and deductive reasoning from other known formulae. E = m*c^2, however, is in fact derived from other formulae (the frequency and wavelength of light equations) and the deductive reasoning that follows from assuming that nothing exceeds the speed of light, leading to the conclusion that space and time are not separate but interact as a unified manifold.
The leading contender for "what is inertia" and "how does it come about" is probably the theory that inertia is simply the result of the relationship between one object and the rest of the universe's mass. The gravitational forces between one object and the rest of the universe "causes" inertia to be the way it is, and all masses that are the same magnitude and are at the same relative velocity all have the SAME inertia despite being located in different parts of the universe. That is, so far as anyone yet can tell. There is a HUGE assumption in the basic homogeneity of the "overall" density of the universe for shape and mass distribution that is probably not true, so theoretically inertia may differ in a very minor way on exact locations. Mass X in Moscow may not really have identical inertia if twin Mass X is located on the Dark Side of the Moon or halfway between Double Stars. The way this concept of inertia makes sense is to conceive of ANY location in the universe as ALSO the CENTER OF MASS of the whole universe. This is just what Cosmologists today say when describing the universe. That's not very "common sense", is it?
"All mechanical quantities can be expressed in terms of three quantities [i.e., mass, length or space, and time]." Apart from this, the concepts like "force" and "energy" are somewhat concocted in an effort to get a good bead on reality. A "force" is just what is said to correspond to what happens when a given mass changes location in space over a certain time. The faster and heavier and farther the change, the greater the "force". Energy is probably real, but I have grave doubts that "force" really means much. It's probably an outdated concept that should be jettisoned entirely.