The elements of thermal chemistry / by M.M. Pattison Muir ; assisted by David Muir Wilson.
- M. M. Pattison Muir
- Date:
- 1885
Licence: Public Domain Mark
Credit: The elements of thermal chemistry / by M.M. Pattison Muir ; assisted by David Muir Wilson. Source: Wellcome Collection.
27/338
![*'••1 the Si have i» J’^'iia,liT(litwen,a been formed ai h this ease the syste have cottslstei of itat Iroside, ^Uen a caai 3Dipf)Sedofcaiiiie,air,a idle, slds of two parts, a change ' the sratem, and a change sm, rk; work being defiBcd c..«tlnninasptein n|cTiratioD adered a^ lke*rlrf ■ilo» ..„ratiB'‘= of the parts of that system, in other words necessitates an acceleration of these parts. The force exerted by one system on the other may be measured by the product of the mass of matter moved and the acceleration produced in that mass. Or, in other words, as mass into velocity = momentum, the force may be measured by the change of momentum which it produces per unit of time. When the mutual actions between the two systems are completed let us suppose that the first system has lost, and the second has gained momentum ; then the first system has also lost energy or power of doing work, and the second system has gained an exactly equal quantity of energy. 8. It is customary to speak of a force as acting on a body or system, but it should not be forgotten that force is merely a convenient name, the use of which helps us to study the mutual actions of bodies, and systems of bodies, which study is the real business of ph}^sical science. Nevertheless in dynamics the actions of forces are measured and compared’without constant reference to the acting and reacting bodies; and the conception implied in the word force has been so much used apart from the conception of two, or more, mutually acting bodies, that the name has almost ceased to be a mere name, and is frequently used as if it represented a real thing. Remembering that behind the word force there is always a reality, we may speak of the application of a force, or even of the work done by a force. SECT. II. §§ 8, 9.] CONSERVATION OF ENERGY. 9. Suppose a man lifts a weight through a space of 3 feet, and places it on a support at that distance from the ground; we have here two mutually acting systems, (a) the man, and (b) the weight and the earth. System (a) does work against the resist- ance of system (6); and when the work is done, system (h) has gained just as much energy as system (a) has lost. For, let the weight be now attached to the machinery of a clock, and let the support be removed ; the weight slowly descends to the ground, and in doing so it communicates energy to the cloek, which energy is spent in overcoming the friction of the various parts, the resistance of the air to the motion of the pendulum &c. Had the weight not been raised from the ground it would have been of no use as a source of energy to the clock. The whole](https://iiif.wellcomecollection.org/image/b28065050_0027.jp2/full/800%2C/0/default.jpg)
