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Is there a good word for change in velocity, but not over time? That is, position is to displacement as velocity is to what?

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@Jim acceleration x time is NOT the definition of velocity. It is equivalent to "change in velocity", which is what Arafinwe is seeking a single word for. –  user16269 Sep 23 '12 at 3:38
@MT_Head I think you have misunderstood the question. He/she wants the difference between the velocity at one moment and the velocity at another moment; just like displacement is the difference between the position at one moment and the position at another moment. I don't believe there IS such a word; I would always just say "change in velocity". It's a really good question though. –  user16269 Sep 23 '12 at 3:56
I should restate: actually, if you subtract velocities, the answer is another velocity. But if we are talking about an actual, real object, the subtraction is meaningless if time is left out; it's an arithmetic exercise with no real-world application. In the real world, objects change their velocity over time. –  MT_Head Sep 23 '12 at 4:31
This might be useful, though... –  MT_Head Sep 23 '12 at 4:34
@Kris - That's a strange question for you to ask... <g> –  MT_Head Sep 23 '12 at 6:37
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4 Answers

up vote 8 down vote accepted

The standard term for a difference of velocities is delta-v.

Quoting Wikipedia (I know, I know!):

In general physics, delta-v is simply the change in velocity. The Greek letter delta is a standard mathematical symbol to represent change (and can be thought of as a fulcrum with a beginning and ending state).
Depending on the situation, delta-v can be referred to as a spatial vector (Δv) or scalar (Δv). In both cases it is equal to the acceleration (vector or scalar) integrated over time:

(Sadly, I don't know enough TeX to include the equations here.)

The term "delta-V" has another, closely-related meaning that brings back a good deal of nostalgia, in this week of watching the Endeavour fly to its final home:

In astrodynamics a Δv or delta-v (literally "change in velocity") is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver.
Delta-v is produced by the use of propellant by reaction engines to produce a thrust that accelerates the vehicle.

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Δv or delta-v are not words! And they do not say "but not over time", either. Should I down vote the answer or vote to close the Q. as off-topic? –  Kris Sep 23 '12 at 6:08
Why are they not words? Their origin is as a mathemtical placeholder, but in many contexts they are, indeed, used like words. "Find the delta-v for this equation". I think it's a good parallel for "displacement". –  Avner Shahar-Kashtan Sep 23 '12 at 6:12
@AvnerShahar-Kashtan Δv is a mathematical notation independent of language -- it's therefore not an English word. –  Kris Sep 23 '12 at 13:21
Δv might a mathematical notation independent of language, but "delta-v" is a English word derived from that notation. "delta", in general, is used to mean "a difference" or "the set of differences". Both "delta" and "delta-v" can be found in some dictionaries (though not all) and in popular usage. –  Avner Shahar-Kashtan Sep 23 '12 at 13:24
@Kris △v is not a word, and thus isn't really what I was looking for. However, this is the first answer which fits the definition; it does mean "but not over time". Furthermore, I think it highly unlikely that there is a single word which fits the definition. Thus, although this is not an English word as such, this will be the accepted answer unless in the rare possibility that an actual word comes along which fits the definition. –  Istvan Chung Sep 23 '12 at 15:13
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Perhaps stasis

condition of balance among various forces; motionlessness

However, I think the analogy would be

position: displacement:: stasis: velocity

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That only really works for lack of change in velocity, not change in velocity. –  Istvan Chung Sep 23 '12 at 3:17
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Several disctionaries define acceleration without reference to direction of change

According to Webster's New World College

Acceleration is a change in the rate of motion, speed or action.

Compact Oxford defines it as

Physics undergo a change in velocity:

Collins says

the rate of increase of speed or the rate of change of velocity

While acceleration is usually used to refer to an increase, it seems possible that it could be expressed as a negative number to show a decrease.

These are clearly general definitions and may or may not reflect the views of the scientific or engineering communities.

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(-1) No, this is incorrect. The problem is not regarding direction, rather it is regarding the time aspect. The question was asking about change, not rate of change. –  Istvan Chung May 28 '13 at 23:36
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The act of accelerating, or the state of being accelerated; increase of motion or action[.]

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(-1) No, this is incorrect. Acceleration is defined as the derivative of velocity with respect to time; I am asking about just the change in velocity. See also the comments on the original question. –  Istvan Chung May 28 '13 at 23:35
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