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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Pressure-gradient force ： ウィキペディア英語版 Pressure-gradient force The pressure-gradient force is the force which results when there is a difference in pressure across a surface. In general, a pressure is a force per unit area, across a surface. A difference in pressure across a surface then implies a difference in force, which can result in an acceleration according to Newton's second law, if there is no additional force to balance it. The resulting force is always directed from the region of higher-pressure to the region of lower-pressure. When a fluid is in an equilibrium state (i.e. there are no net forces, and no acceleration), the system is referred to as being in hydrostatic equilibrium. In the case of atmospheres, the pressure gradient force is balanced by the gravitational force, maintaining hydrostatic equilibrium. In Earth's atmosphere, for example, air pressure decreases at increasing altitudes above Earth's surface, thus providing a pressure gradient force which counteracts the force of gravity on the atmosphere. == Formalism == Consider a cubic parcel of fluid with a density $\backslash rho$, a height $dz$, and a surface area $dA$. The mass of the parcel can be expressed as, $m\; =\; \backslash rho\; \backslash cdot\; dA\; \backslash cdot\; dz$. Using Newton's second law, $F\; =\; m\backslash cdot\; a$, we can then examine a pressure difference $dP$ (assumed to be only in the $z$-direction) to find the resulting force, $F\; =\; -\; dP\; \backslash cdot\; dA\; =\; \backslash rho\; \backslash cdot\; dA\; \backslash cdot\; dz\; \backslash cdot\; a$. The acceleration resulting from the pressure gradient is then, $a\; =\; \backslash frac\; \backslash frac$. The effects of the pressure gradient are usually expressed in this way, in terms of an acceleration, instead of in terms of a force. We can express the acceleration more precisely, for a general pressure $P$ as, $\backslash vec\; =\; \backslash frac\; \backslash vec\backslash nabla\; P$. The direction of the resulting force (acceleration) is thus in the opposite direction of the most rapid increase of pressure. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pressure-gradient force」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.