Mechanical energy

“ We call the energy of motion kinetic energy. Potential energy is the energy a body has in virtue of its position—it is its capacity, in other words, to acquire kinetic energy, as in the case of a stone resting on the edge of a cliff.
Energy may assume different forms; one kind of energy may be converted directly or indirectly into some other form. The energy of burning coal, for example, is converted into heat, and from heat energy we have mechanical energy, such as that manifested by the steam-engine. In this way we can transfer energy from one body to another. ”

“ At the same time a part of this electrical motion is being converted into a heat motion of the atoms, and finally it all returns to heat unless some of it is stored up somewhere as potential energy. If the current has driven a motor that has wound up a weight, a part is stored up in the weight, which has the ability to do work if it is allowed to run down. If it drives machinery as it runs down, the mechanical motion is the expression of the stored energy. ”

“ In the elements of physics it is nowadays taught that mechanical work may be transformed into heat, and reciprocally that heat may be transformed into mechanical work. Friction, impact, pressure, and expansion destroy or annihilate the mechanical energy communicated to a body or to the organs of a machine. With the disappearance of motion we note the appearance of heat. ”

“ All the energy, however, set free by oxidation from the fuel is not transformed into mechanical energy. A part, a large part, as much as eighty-eight per cent of it, is set free simply as heat, which, I need hardly say, is of no use to the engineer. The same kind of reasoning guides engineers in the construction of all kinds of engines, and they are always striving to get as much as possible of the energy of the fuel transformed into mechanical energy. ”

“ In the first case, ordinary mechanical or translational energy is spent as friction, an amount measurable in foot-pounds, and the factors we [Pg 98] know, a pressure into a distance. If the surface be of the same kind of molecules, the whole energy is spent as heat, and is presently radiated away. If the surfaces are of unlike molecules, the product is a compound one, part heat, part electrical. What we have turned into the machine we know to be a particular mode of motion. ”

“ This we do when we speak of mechanical energy as the energy involved in a body having a translatory motion; also, when we speak of heat as a vibratory, and of light as a wave motion. To speak of energy without stating or implying these distinctions, is to speak loosely and to keep far within the bounds of actual knowledge. To speak thus of a body possessing energy, or expending energy, is to imply that the body possesses some kind of motion, and produces pressure upon another body because it has motion. ”

“ In examining the concept of energy, we start with purely mechanical energy. In isolated mechanical systems, in which there are only conservative forces, the sum of kinetic and potential energy is constant. The kinetic energy may be defined as ∑ 1⁄2 mv2, formed for all parts of the body. The potential energy is determined by the position of the parts of the system, and has physical significance only [Pg 108] with reference to a datum position, that is, only changes of potential energy have meaning in terms of operations. ”

“ The system consisting of the earth and the pound therefore possesses an amount of energy which depends on the relative positions of its two parts, on account of the latent physical connexion existing between them. In most mechanical systems the working stresses acting between the parts can be determined when the relative positions of all the parts are known; and the energy which a system possesses in virtue of the relative positions of its parts, or its configuration, is classified as “potential energy,” to distinguish it from energy of motion which we shall presently consider. ”

“ This followed from his theorem that the total mechanical energy of a flowing liquid, comprising the energy associated with fluid pressure, the gravitational potential energy of elevation, and kinetic energy of fluid motion remains constant; that is, the mechanical energy is conserved. ”

“ Thus, when a bullet is shot against a target, before it strikes it has what we call mechanical energy, and we measure that in foot-pounds; after it has struck the target, the transformation is into heat, and this has its mechanical equivalent, but is not called mechanical energy, nor are the motions which embody it similar. The mechanical ideas in these phenomena are easy to grasp. ”

“ The same thing happens with the electricity. This electricity which travels at the speed of 163,000 miles a second, when it comes into frictional relation with its conductor pushes aside the molecules of the metal, and here the mechanical energy is magically transformed into heat. ”

“ Among other examples, that of the coiled spring which is unwound is particularly suitable for showing this fundamental character of the idea of mechanical energy, an idea which is the clearest of all. Machines are only transformers and not creators of mechanical energy. ”

“ Energy is like water-power—potential in the lake, actual in the waterfall or river.
Any mechanism capable of causing one form of energy to pass into another is a transformer of energy. A steam engine is a transformer of energy, changing caloric energy into mechanical work. An electrical machine is a transformer of energy, converting mechanical motion into a current of electricity, whilst an electro-motor changes the movement of electrons into mechanical movement. ”

“ A mechanical device, however ingenious may be the construction, or however accurate the workmanship, can never possess what is called perpetual motion. It is needless to enter into details of any proposed contrivance of wheels, of pumps, of pulleys; it is sufficient to say that nothing in the shape of mechanism can work without friction, that friction produces heat, that heat is a form of energy, and that to replace the energy consumed in producing the heat there must be some source from which the machine is replenished if its motion is to be continued indefinitely. ”

“ In the production of mechanical effect from heat, gaseous fuel also presents most striking advantages, as will appear from the following consideration. When we have to deal with the question of converting mechanical into electrical effect, or vice versa, by means of the dynamo-electrical machine, we have only to consider what are the equivalent values of the two forms of energy, and what precautions are necessary to avoid losses by the electrical resistance of conductors and by friction. ”

“ I then attempt to show that, if to these premises we add the assumption of the atomic constitution of matter, the mechanical theory necessarily involves four distinct propositions, relating severally to the equality, inertia, and inelasticity of the atoms or ultimate molecules and the essentially kinetic character of what is now universally termed energy. ”

“ An animal, like a machine, only transforms its energy. Lavoisier's Guinea-pig placed in the calorimeter gave as accurate a heat-return for the energy it had absorbed in its food as any thermic engine would have done. But the parallel goes further. The mechanical work of an engine is measured by the loss of its heat and not of its substance. So the mechanical or intellectual work of a living being is measured by the amount of food rather than the amount of tissue which is burned. The energy evolved daily by the human body would raise it to a height of about six miles. ”

“ In this wider field we cannot assert that mechanical (or available) energy is never lost, for it may be degraded into thermal energy; but we can use the principle that on the other hand it can never spontaneously increase. If this were not so, cyclic processes might theoretically be arranged which would continue to supply mechanical power so long as energy of any kind remained in the system ”

“ Not only is kinetic transformed into potential energy and vice versa, but the different forms of kinetic energy (heat, light, electricity, sound, and mechanical motion) are readily transformed the one into the other. With suitable devices, mechanical motion can be changed into heat, sound, or electricity; heat into motion and light; and electricity into all the other forms of energy. ”

“ If the vibratory movement is continuous, the rotatory velocity of the system will steadily increase in value. Energy once stored in the radial springs can only be released by the return movement of the masses and in the form of energy of rotation; the nature of the mechanical machine is, in fact, such that if any incremental energy is applied to the displacement of the masses against the radial springs, it can only be returned in this form of energy of motion. ”

“ Spheres, or bodies of any other form, if reduced in dimensions to 1⁄760,000,000th, would as reservoirs of energy gain 760,000,000-fold. Thus we open a door of explanation regarding the stupendous contrast between chemical energy and mechanical work. Chemical processes are exerted by molecules and atoms, mechanical work takes place among masses comparatively enormous in bulk. ”

“ Further measurements on the transformations of other forms of energy, chemical energy and electrical energy, have shown that Joule's law of equivalents is general, and that the quantity of mechanical work represented by any form of energy remains undiminished after transformation, whatever the nature of that transformation.
Energy presents itself to us under two forms, potential and actual. Potential energy is slumbering energy, energy localized or locked up in the body. ”

“ If the electrical energy be transformed into mechanical of any sort, no one thinks of calling the latter electrical because of its antecedent. If electrical motions be transformed into ether actions of any kind, why should we continue to speak of the transformed motions or energy as being electrical? Electricity may be the antecedent, [Pg 115] in the same sense as the mechanical motion of a bullet may be the antecedent of the heat developed when the latter strikes the target ”

“ III. THE PRINCIPLE OF ENERGY IN PHYSICS.
We can know only from experience that mechanical processes produce other physical transformations, or vice versa. The attention was first directed to the connexion of mechanical processes, especially the performance of work, with changes of thermal conditions by the invention of the steam-engine, and by its great technical importance. Technical interests and the need of scientific lucidity meeting in the mind of S. Carnot led to the remarkable development from which thermodynamics flowed. ”

“ To these powers, which are unquestionable and distinct, it is usual to add vital force, of which, however, it is difficult to speak as a whole; but one member of our vital energies, the nerve-force, allied to electricity, fully deserves to rank in the correlation.
Taking the one mechanical force, and those three of the molecular named heat, chemical force, electricity, there has now been established a definite rate of commutation, or exchange, when any one passes into any other. ”

“ So we find that the three possible solutions of the great problem of increasing human energy are answered by the three words: food, peace, work. Many a year I have thought and pondered, lost myself in speculations and theories, considering man as a mass moved by a force, viewing his inexplicable movement in the light of a mechanical one, and applying the simple principles of mechanics to the analysis of the same until I arrived at these solutions, only to realize that they were taught to me in my early childhood. These three words sound the key-notes of the Christian religion. ”

“ Electrical action, chemical power, crystalline aggregation, the expansive force of heat, and many other known agencies, are in constant operation to prevent it. It must, however, be remembered, that each and every atom constituting a mass may be so suspended between the balanced forces, that it may be regarded as relatively at rest.
Theory imagines Motion as producing Force—a body is moved, and its mere mechanical change of place is regarded as generating heat; and hence the refinements of modern science have advanced to the conclusion that motion and heat are convertible. ”

“ This is eminently a case in which energy is transmitted by matter in motion. The moving material comprises the piston, piston-rod, and connecting-rod, which are, one and all, endowed with both cohesive and kinetic energy qualities, and form together the transmission machine. So long as the piston is at rest only one aspect of the work energy of the gas is apparent, namely, the pressure aspect, but immediately motion and transmission take place, both aspects are presented. ”

“ In this particular case, the energy compass of the machine is restricted by certain physical properties of the material, by the maximum value of these cohesive or elastic forces called into play in distortion. These forces are themselves the evidence of energy, of that energy by virtue of which the material possesses and maintains its coherent form. In this case this energy is also the factor controlling the transformation, and appears as a separate and distinct incepting agency. ”

“ Thus, chemical action (oxidation in the animal system) supplies muscular power, this sets in motion a Gramme machine, the motion is converted into electricity, the electricity produces the electric light, and light causes chemical action, and with this the cycle is complete. In the steam engine heat is converted into mechanical force, and many cases will present 804themselves to the reader’s mind in which mechanical actions give rise to heat. The doctrine of a mutual dependence and convertibility among all the forms of force was first definitively taught in England by Mr. ”