Not that it matters much...
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On with the hair splitting, it isn't or hasn't been for many decades a valid engineering or scientific term if that suits you better. According to the scientific guidelines, you are always accelerating in one direction or the other. Never saw it myself but neither "decelerate" or any of it's derivatives was proper to use in engineering and scientific writing. Figured if I didn't mention it, somebody else would!
Once upon a time I wrote and edited scientific papers. "decelerate" was a term we didn't use. A quick look turned up the information below. More digging would turn up reams of documentation I suspect. Been over twenty years since I have done any serious technical writing. Back then we couldn't use the word. Kinda like the words "ain't" or "kinda"! Everybody knows what we mean but we still don't use the word in technical writing, or didn't in my day.
According to the protocols of the day, there was positive and negative acceleration, one or the other if something was changing speed. Deceleration wasn't a proper term. Using it was a quick way to get your writing redflagged.
There are entire sets of dictionaries. Always wanted a set if I ran across one cheap. I have seen sets of dictionaries that were as bulky as the Encyclopedia Britannica. I suspect there are lots of words in there that aren't considered proper.
I'll continue to use "decelerate" on a pool forum when convenient. If I ever send another proposal to NASA I still won't use it, or "won't" either!!(grin) In technical terms deceleration is negative acceleration, a bit of knowledge few if any pool players will ever need to know!
Now on to important things: How many angels can stand on the head of a pin?
Hu
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No such thing as decelerating only acceleration?
We tend to avoid the term "decelerating," because the temptation is to assign that term to any problem where the acceleration is negative. Just because the acceleration is negative doesn't necessarily mean the thing is slowing down. If the velocity is also negative, then a negative acceleration means it's speeding up. More on reddit.com