Of course I meant readers to think that I was referring to the instantaneous speed.
The primary definition of "then" (try dictionary.com for instance). is "at that time". The defintion of "thereafter' is "from a specific time onward".
You misintpreted "then" to mean "thereafter". Don't criticise my physics when your English is at fault.
IXP
OK from
www.dictionary.com
then Audio pronunciation of "then" ( P ) Pronunciation Key (thn)
adv.
1. At that time: I was still in school then. Come at noon; I'll be ready then.
2. Next in time, space, or order; immediately afterward: watched the late movie and then went to bed.
3. In addition; moreover; besides: It costs $20, and then there's the sales tax to pay.
4. Used after but to qualify or balance a preceding statement: The star was nervous, but then who isn't on the first night of a new play.
5. In that case; accordingly: If traffic is heavy, then allow extra time.
6. As a consequence; therefore: The case, then, is closed.
Of course, I used the complete Oxford English dictionary, which puts example 2 first. (At least the one on my shelf behind me does.)
The "at that time" is exemplified by past tense statements. None of the examples refer to a moment in time, which would be implied by "at that instant" or "instantanious".
Your meaning, as you wrote it, best fits #2: "next in time" ... the object falls 16 feet and then (next in time) travels at 32 fps.
Both interpretations have their troubles, so I look to the rest of the statement for context. This helps seperate the meanings. This is important as I guessed you were speaking informally and with some emotion. Of course, I could have just used a dictionary and then opted for the most common usage - but note:
my dictionary agrees with me.
To my (English) mind, when you also described the motion as "travel", there was the implication of a journey - ongoing motion. Also from dictionary.com:
trav·el Audio pronunciation of "travel" ( P ) Pronunciation Key (trvl)
v. trav·eled, or trav·elled trav·el·ing, or trav·el·ling trav·els or trav·els
v. intr.
1. To go from one place to another, as on a trip; journey.
2. To go from place to place as a salesperson or agent.
3. To be transmitted, as light or sound; move or pass.
4. To advance or proceed.
5. To go about in the company of a particular group; associate: travels in wealthy circles.
6. To move along a course, as in a groove.
7. To admit of being transported without loss of quality; Some wines travel poorly.
8. Informal. To move swiftly.
9. Basketball. To walk or run illegally while holding the ball.
See what I mean? There is a definate sence of continued motion in most of these uses.
However, I am assured by other Yanks that the word "travel" is synonymous with "move" for informal speach. For the sake of peace, I am prepared to accept that we are being divided by a common language here

However, consider the following:
I got in my car, headed out onto the highway, and traveled at 50kmph.
Would you expect folk to consider "then" to mean "at that time" or "after 'I headed out'"? Would you expect folk to consider that I had an instantanious speed of 50kmph (and that I was still accelerating) or that I traveled at a constant 50kmph (or perhaps an average 50kmph - with minor variations)? How do you think most people would understand that statement?
In fact - lets ask them: Most People? How would you understand that statement? Don't be shy - we can learn something about the international understanding of English here and maybe help improve communication on this board and in our lives. Anyone?
Meanwhile: a word about dictionaries ...
Dictionaries come in two main flavours, there are the definitive ones and the descriptive ones.
Most older dictionaries were difinitive. They "defined" words... and thus acted as authorities to tell you how to "correctly" use the word. The original Websters Dictionary was like this. The whole point was to teach English to immegrants to the USA.
However, this use of dictionaries has become unfashionable from about the mid 20th century ce. Modern dictionaries are descriptive: the describe how the word is used. The Cambridge Modern English Dictionary is an example of this.
www.dictionary.com is like this also - they do not provide "definitions" of words. Instead they give examples of the usage of words.
This sort of thing makes sence. The main reason you want to look up a word is not to find out how it is supposed to be used, but to get an idea of how a writer has used it. The listed usage is usually in the order that some sort of research suggests is most common usage when the dictionary goes to print.
So -
www.dictionary.com says the following:
gay Audio pronunciation of "gay" ( P ) Pronunciation Key (g)
adj. gay·er, gay·est
1. Of, relating to, or having a sexual orientation to persons of the same sex.
2. Showing or characterized by cheerfulness and lighthearted excitement; merry.
3. Bright or lively, especially in color: a gay, sunny room.
4. Given to social pleasures.
5. Dissolute; licentious.
n.
1. A person whose sexual orientation is to persons of the same sex.
2. A man whose sexual orientation is to men: an alliance of gays and lesbians.
... see what I mean. The number one usage is not the original usage nor the only usage. It would be the most likely usage in a modern work, given no additional information. To be sure of our meaning, we must look to the rest of the sentance for a guide, then make use of the examples to provide a reference. This is called "context".
Happy Hacking.
Simon