4

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} (speed), 度{ど}(degree).

Can I guess anything based on the words etymology, or is the nuance between them is more subtle ?

9

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

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

3

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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