Volume 1
A course of lectures on natural philosophy and the mechanical arts / by Thomas Young.
- Thomas Young
- Date:
- 1845
Licence: Public Domain Mark
Credit: A course of lectures on natural philosophy and the mechanical arts / by Thomas Young. Source: Wellcome Collection.
Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.
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![mentum as would have been obtained by the immediate action of an equal I force on the body to be moved. An elastic ball of 2 ounces weight, moving with a velocity of 3 feet in a ■ second, possesses an energy, as we have already seen, which may be ex- 1 pressed by 18. If it strike a ball of 1 ounce which is at rest, its velocity i will be reduced to 1 foot in a second, and the smaller ball will receive a I velocity of 4 feet: the energy of the first baU wiU then be expressed by 2, i and that of the second by 16, making together 18, as before. The mo- j mentum of the larger baU after collision is 2, that of the smaller 4, and the j sum of these is equal to the original momentum of the first ball. Supposing the magnitude of an elastic body which is at rest to be infinite, it will receive twice the momentum of a siiiall body that strikes it; but its velocity, and consequently its energy, wiU be inconsiderable, 1 since the energy is expressed by the product of the momentum into the ' velocity. And if the larger body be of a finite magnitude, but stiU much greater than the smaller, its energy will be very small; that of the smaller, which rebounds with a velocity not much less than its original velocity, being but little diminished. It is for this reasoir that a man, having a heavy an\dl placed on his chest, can bear, without much inconvenience, the blow of a large hammer striking on the anvil, while a much slighter blow of the hammer, acting immediately on his body would have fractured his ribs, and destroyed his life. The anvU receives a momentum nearly twice as great as that of the hammer ; but its tendency to overcome the strength of the bones and to ci-ush the man, is only proportional to its energy, which is nearly as much less than that of the hammei’, as four times the weight of the hammer is less than the weight of the anvil. Thus, if the weight of the hammer were 5 pounds, and that of the anvil 100, the energy of the anvil would be less than [only] one fifth as great as that of the hammer, besides some further diminution, on account of the want of perfect elas- ticity, and from the effect of the larger surface of the anvil in dividing the pressure occasioned by the blow, so as to enable a greater portion of the chest to cooperate in resisting it. When a body strikes another in a direction which does not pass through its centre of gi'avity, the effect produced involves the consideration of rotatory motion, since, in tliis case, the body is made to revolve on an axis. But this can never happen when the body is spherical, and its surface perfectly polished ; since every impulse must then be perpendicular to the surface, and must consequently be directed to the centre of the body. If the motion of a ball which strikes another is not directed to its centre, the surface of contact must be oblique with respect to its motion, and the second ball will only receive an impulse in a direction perpendicular to this surface, while the first I'eceives, from its reaction, an equal impulse in I a contrary direction, which is combined with its primitive motion. The I magnitude of this impulse may be determined by resolving the motion of I the first ball into two parts, the one parallel to the surface of contact, and I the other perpendicular ; the first part remaining always unaltered, the second being modified by the collision. If, for example, the balls were equal, this second part of the motion would be destroyed, and the remain-](https://iiif.wellcomecollection.org/image/b21301840_0001_0095.jp2/full/800%2C/0/default.jpg)
