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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 speed ： ウィキペディア英語版 speed In everyday use and in kinematics, the speed of an object is the magnitude of its velocity (the rate of change of its position); it is thus a scalar quantity.〔 This is the likely origin of the speed/velocity terminology in vector physics.〕 The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero. Like velocity, speed has the dimensions of a length divided by a time; the SI unit of speed is the metre per second, but the most common unit of speed in everyday usage is the kilometre per hour or, in the US and the UK, miles per hour. For air and marine travel the knot is commonly used. The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in a vacuum ''c'' = metres per second (approximately or ). Matter cannot quite reach the speed of light, as this would require an infinite amount of energy. In relativity physics, the concept of rapidity replaces the classical idea of speed. ==Definition== The Italian physicist Galileo Galilei is credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time.〔Hewitt (2006), p. 42〕 In equation form, this is :$v\; =\; \backslash frac,$ where $v$ is speed, $d$ is distance, and $t$ is time. A cyclist who covers 30 metres in a time of 2 seconds, for example, has a speed of 15 metres per second. Objects in motion often have variations in speed (a car might travel along a street at 50 km/h, slow to 0 km/h, and then reach 30 km/h). In mathematical terms, the speed $v$ is defined as the magnitude of the velocity $\backslash boldsymbol$, that is, the derivative of the position $\backslash boldsymbol$ with respect to time: :$v\; =\; \backslash left|\backslash boldsymbol\; v\backslash right|\; =\; \backslash left|\backslash dot\; \backslash right|\; =\; \backslash left|\backslash frac\backslash right|\backslash ,.$ If $s$ is the length of the path travelled until time $t$, the speed equals the time derivative of $s$: :$v\; =\; \backslash frac.$ In the special case where the velocity is constant (that is, constant speed in a straight line), this can be simplified to $v=s/t$. The average speed over a finite time interval is the total distance travelled divided by the time duration. 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「speed」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.