I am trying to describe the evolution of a motion which is composed of smooth parts called "free flights" and instantaneous impacts. For example, consider a bouncing ball: its motion is a succession of free flights, separated with impacts (when the ball touches the ground).

I would like to refer to two time-related quantities: the duration of free flights, and the dates of impacts. The word "time" can refer to a length of time, or an instant (a date). To raise the ambiguity, I have chosen the following terminology:

  • "free-flight durations" describes a length of time between two impacts;
  • "impact times" describes the date of impact.

Is this correct and would "impact instants" or "impact dates" be better? I am open to other suggestions too, but I want to avoid "period" which I am already using to qualify repetitiveness.

  • 1
    I think you're fine. Although I'd take the 's' off "free flights" It's just "free-flight durations" and then you can use TOIs or Times of Impact for a list of times. A single time is just a Time of Impact.
    – Jim
    Jan 5, 2016 at 22:12
  • @Jim. Thank you. I'll edit my question accordingly as I did not mean to add the "s". Out of curiosity, would "impact instants" have been correct?
    – anderstood
    Jan 5, 2016 at 22:18
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    According to the International Electrotechnical Vocabulary, "In common language, the word “time” is used with several different meanings. In technical language, however, more precise terms, e.g. date, duration, time interval should be used."
    – The Photon
    Jan 5, 2016 at 22:45
  • 2
    @StoneyB, in technical usage, period normally refers to the duration of each repetition of a periodic (repeating) behavior. For example, "the period of a sine wave" or "the orbital period [of a planet or moon]". In fields (like physics or engineering) where this usage is common, using period to describe a non-periodic event could be confusing.
    – The Photon
    Jan 5, 2016 at 22:52
  • 1
    @StoneyB- I actually interpreted OP’s question to be about durations. For example I might have expected something like “The complete flight consisted of 3 periods with free-flight durations of 1.5, 2.1, and 15.2 seconds respectively. Impacts occurred at 0:00:01.5, 00:00:3.6, and 00:00:18.8 ( with start of flight at 00:00:00)
    – Jim
    Jan 5, 2016 at 22:54

1 Answer 1


Interval, mentioned in a comment, is indeed the technical word for a time span and is used both as an event and as a measure:

"The free-flight interval begins/began at 1:02:30."

"The free-flight interval is/was 4 seconds." (No need to say "lasts/lasted 4 seconds.")

"The 4-second free-flight interval ends/ended at 1:02:34."

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