# Can the difference in velocities be called as “velocities difference”?

Tortoises walk with the speed of 0.133 m/s. Rabbits run with the speed of 20 m/s. The difference in their speed is Δv = 20 - 0.133 ≈ 20 m/s. Can I say the speeds difference/velocities difference of them is almost 20 m/s? Can the difference in velocities be called as "velocities difference"? How can I refer to this Δv concept later on without having to mention the objects involved? I also don't want to confuse Δv = vobject 1 - vobject 2 with the Δv = vfinal - vinitial.

• There's no need for a label when you can say their difference in velocity. their velocity difference is stylistically clumsy and saves you only two letters and a space. – Tᴚoɯɐuo Mar 22 '18 at 21:09
• As a general rule (as Jasper pointed out) when we make a compound noun, the first one is singular: horse race, not horses race, even though there is more than one. – stangdon Mar 23 '18 at 0:07
• @stangdon but why is "collocation" in collocations dictionary plural? – Ooker May 7 '18 at 13:32
• @Ooker - A good question. All I can say that it is a general rule, and there are a few exceptions. – stangdon May 7 '18 at 16:06
• @stangdon are you surprised of the exception, or you have known it before? Is there a rule for exceptions? – Ooker May 7 '18 at 18:24

## 2 Answers

"Speed difference" is okay in this context, but "velocities difference" has two errors:

• The difference is in speed (rate of movement, regardless of direction), not velocity (a speed in a particular direction).
• Most attributive nouns are singular.

There is also a significant figures problem. 20 (give or take 5) minus 0.133 gives a final answer that is practically indistinguishable from 20 (give or take 5).

The difference between two velocities (at a given point in time) is a "relative velocity". When discussing relative velocities, one needs to be clear which object's velocity is being subtracted. For example, are you talking about the velocity of the rabbit relative to the turtle, or of the turtle relative to the rabbit?

• But can I say "velocity difference" if I do want to count it as vectors? – Ooker Mar 22 '18 at 15:11
• @Ooker -- Yes, you can say "velocity difference" if you are comparing vectors. – Jasper Mar 22 '18 at 15:35
• But regarding Tᴚoɯɐuo's answer and comment, "velocity difference" is clumsy. Do you think so? – Ooker Mar 23 '18 at 0:02

Setting scientific definitions of velocity aside, you would not say velocities difference but the difference in their velocity or their difference in velocity.

The difference in their velocity is almost 20 meters per second.

Their difference in velocity is...

The pattern is `the difference in (their) {measurable quality}` or `their difference in {measurable quality}`.

The difference in their height.

The difference in their weight.

The difference in their brightness.

etc

• Actually I need a concise term because later on my article I need to reuse it – Ooker Mar 22 '18 at 15:09
• You're looking for a label. Sorry. This site is about English, not about labels. – Tᴚoɯɐuo Mar 22 '18 at 17:48
• but I think this label is about English also? It's nothing different to a word-request? – Ooker Mar 22 '18 at 23:58
• Actually, it is more about scientific writing conventions which place greater emphasis on labeling everything than on striving to be idiomatic. Labels conventionally eschew prepositions and rely on noun-adjunct modifiers: velocity difference versus difference in velocity. – Tᴚoɯɐuo Mar 23 '18 at 11:13
• Concision in prose is not achieved by replacing difference in velocity with velocity difference. You could replace with the speed of with the word at. "Rabbits run at 20 m/sec". – Tᴚoɯɐuo Mar 24 '18 at 9:35