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From what I understood, both **速さ{はやさ}** and **速度{そくど}** means "speed". Some
say that they are synonyms (just like _speed_ and _velocity_ in English), but
I guess there is a nuance between them, isn't it ?

**速さ{はやさ}** comes from the adjective **速い{はやい}** , like other adjectives (
**長い{ながい}** : **長さ{ながさ}** , **深い{ふかい}** : **深さ{ふかさ}**...), while **速度{そくど}**
is the combination of two kanjis :
[**速{そく}**](https://en.wiktionary.org/wiki/%E9%80%9F#Kanji) (speed),
[**度{ど}**](https://en.wiktionary.org/wiki/%E5%BA%A6#Japanese)(degree).

Can I guess anything based on the words etymology, or is the nuance between
them is more subtle ?
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word-choice words etymology word-usage wago-and-kango
Is there any difference between 速さand 速度?
I would say there is no difference, at least in everyday language. Maybe 速さ
has a slightly more casual feel to it... at least I see more myself using 速さ
than 速度 in a daily conversation.

Now, 速度 is **velocity** and 速さ **speed**. That means that in the field of
physics, they _are_ indeed different, namely:

  * velocity is a vector, including not only a value but a direction
  * speed is a scalar quantity (the magnitude of the velocity)

edit: just confirmed on
[wikipedia](https://ja.wikipedia.org/wiki/%E9%80%9F%E5%BA%A6)

> 日常語としての速度と速さはほとんど区別なく使われている [...] 対して、物理学においては、速さ（英語:
> speed）と速度（velocity）を厳密に区別する
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As a single word, Nicolas Couvrat is true.  
But when including combination with 速い/遅い, there is a difference.

速さ includes the meaning of 'fast' (<=> 遅さ), whereas 速度 is neutral.  
So we say 速度が遅い, but we don't say 速さが遅い。

Example:

1.「見てよ！ この電車の速さ！」= "Look! How fast this train moves!"

2.「見てよ！ この電車の速度！」= "Look! This train's speed!"

This "速度" of 2. can mean slow (like talking about legacy train in rural area).
If you want to explicitly say slow

「見てよ！ この電車の遅さ！」= "Look! How slow this train moves!"
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