Meaning of velocity - mass noun vs countable noun

Employment / acceptance of an indefinite article isn't a good test for a count usage.

'She has a working knowledge of Spanish'. 7 working knowledges?

Here, though, the second sentence means

'The body has, in addition, vibration[al] velocity.'

Just as a body may possess kinetic energy of translation, KE of rotation, and, in addition, KE of vibration, it will correspondingly have associated translational and rotational velocities, and one assumes what may be labelled a vibrational velocity. Though 'velocities' is commonly used hereabouts, the use of numerals before the word is rare.

Velocity is probably best regarded as non-count in say Velocity is a vector quantity indicating not only speed but also direction (though the plural form isn't outlandish).

The first sentence sounds strange, and probably defaults to The body has vibration velocity over and above [what has been specified].

What is the difference between the following sentences? 1 The body has additional vibration velocity. vs. 2 The body has an additional vibration velocity.

It is perhaps easiest to discard the adjectives:

1 The body has velocity. -> the body possesses the abstract attributes of speed and direction. Here, we are saying that the body possesses speed and direction “in general”

vs.

2 The body has a velocity. -> the body possesses an example of a certain speed and an example of a certain direction. We are being a little more specific - the figures for each are real.

If we return to “vibration velocity” in 1, this means “general velocity that is associated with general vibration”. It is an uncountable noun phrase (comprised of two nouns used uncountably), which is a subset of the uncountable noun “velocity”. .

2 In, “a vibration velocity”, vibration is an uncountable noun (as above, it acts adjectivally), but “velocity” is countable and the sentence is to be understood as “The body has an example of an certain velocity associated with vibration in general.

Now for “additional”: In 1, it means greater than that in other bodies. In 2, it means one more “vibration velocity” than in other bodies.