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In English, there's a technical difference between the words speed and velocity that appears when you study introductory physics: velocity is "vector," meaning it has both a magnitude and a direction, while speed is "scalar," meaning it only has a magnitude. Thus, if I simply want to communicate how fast someone is going, I'd say "his speed is 30 meters/second." If I want to communicate direction as well, I'd say "her velocity is 30 meters/second toward the North."

Is this distinction maintained in Spanish as well, with the words rapidez (speed) and velocidad (velocity), or some other pair of words?

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    Also note that English has a third related noun: quickness. That will generally only correspond to rapidez (the -ness English ending is analogous to -ez Spanish, "quality of [adj.]") – user0721090601 Jul 1 '15 at 0:11
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Basically, yes.

However, that's a very restricted technical (and not very important) usage. In common usage, both terms are approximately equivalent. Also, "rapidez" is often used to mean "high velocity" or "high rate".

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    I would say that common usage is almost the opposite as in English: we would never say su rapidez es 30 km/h, but su velocidad es 30 km/h. I cannot think of an example, outside a technical use in Physics, where rapidez is quantified. – Gorpik Jun 30 '15 at 18:11
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    I'm with @Gorpik here. In Spain, velocidad is used as speed all the time in normal speech (límite de velocidad, velocidad de la luz, velocidad de crucero, exceso de velocidad...). I don't even remember using different words for speed and velocity when learning Physics. I think we used velocidad for speed and "vector velocidad" for velocity. – MikMik Jul 6 '15 at 11:48
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    Additionally, a common term in Physics for scalar velocity is celeridad. I've seen it more often than rapidez in that context. Though celeridad is never used outside the technical field. – Gorpik Jul 7 '15 at 9:14
  • I'm downvoting this answer: I've never said, heard or read something such as "una rapidez de 30 m/s" in my life, neither in technical contexts (I'm a physicist) nor in common life. – Charo May 23 '20 at 18:58
  • @Gorpik: I've a PhD in Physics and I can assure you that "rapidez" is neither used that way in Physics. – Charo May 24 '20 at 7:02
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No, in Spanish we do not employ "rapidez" in the same way as English word speed is used with the sense you mention in the question.

I'm a physicist and I would use the term "velocidad" either for the vector or for the modulus (or, as you call it, magnitude) of the vector: in case of possible ambiguity, I would say "el módulo de la velocidad", but in many contexts I would produce sentences such as "la velocidad del móvil es de 25 m/s" without any problem.

Another example: the "speed of light" is called in physics "la velocidad de la luz".

Having a look at Google Books, I've found some physics texts that use "celeridad" to refer to the modulus of the velocity vector. Personally, I have never used such word with that meaning and I've not seen it in the Spanish texts I've used (either as a student or in my professional life). Many other physicists I known do not use it either.

In common life, we also use "velocidad" for the modulus of the vector (average or instantaneous velocity). We do not use "rapidez" to express such concept.

"Rapidez" is mainly used to express the quality of being fast or acting quickly. For example, here is a sentence from the book Yo he sido casada by Rafael López de Haro,

Le agradecí a Susana la rapidez con que vino en mi ayuda

that I would more or less translate as "I thanked Susana for coming to help me so quickly".

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Despite they are both different things, being one a vector and the other one a scalar, you get to find a lot of people saying velocidad or rapidez for the same thing.

Clearly they are both different, but spanish speakers normally work with those having not much distinction on them.

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  • This doesn't correspond to the technical vocabulary being asked in the question. And, even in common life, we do not use "rapidez" to say things such as "una rapidez de 60 km/h". – Charo May 23 '20 at 20:25
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In the usual context, the daily use, we don’t differentiate both terms; we use velocidad for velocity and speed.

Even it’s different, Only in the technical language we use two terms Velocidad for speed (scalar) and Celeridad for velocity (vector).
In the technical field It may looks confuse because velocity looks/sounds more similar to velocidad.

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    No es así: véase mi respuesta para más detalles. En todo caso, creo que en algún país se usa "celeridad" como el módulo del vector velocidad: yo soy física y jamás lo he usado (en España), como nunca he utilizado "vecindad" para traducir "neighbourhood", "rango" en lugar de "recorrido" de una función o "mapeo de A en B" en lugar de "aplicación de A en B" (A y B son dos conjuntos). – Charo May 23 '20 at 22:20
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They are not the same thing. Velocidad is a vector quantity, (it has a direction). Rapidez is a scalar quantity. So velocidad might be more like velocity and rapidez to speed. They can't be replaced between them neither in Spanish or English.

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