Is there a good word for change in velocity, but not over time? That is, position is to displacement as velocity is to what?
The standard term for a difference of velocities is delta-v.
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.
Several disctionaries define acceleration without reference to direction of change
According to YourDictionary:
Acceleration is a change in the rate of motion, speed or action.
Oxford defines “accelerate” as
Physics undergo a change in velocity
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.
condition of balance among various forces; motionlessness
However, I think the analogy would be
position: displacement:: stasis: velocity
Acceleration, as defined in Wiktionary:
- The act of accelerating, or the state of being accelerated; increase of motion or action…
- The amount by which a speed or velocity increases …