Timeline for Why is speed defined like it is?
Current License: CC BY-SA 3.0
14 events
| when toggle format | what | by | license | comment | |
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| Jul 31, 2017 at 13:29 | history | edited | FGSUZ | CC BY-SA 3.0 |
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| Jul 31, 2017 at 11:14 | comment | added | Sophie Swett | Wikipedia suggests that the definition "speed is distance divided by time" was discovered by Galileo: en.m.wikipedia.org/wiki/Speed#Historical_definition I do find it hard to believe that the Ancient Greeks didn't figure it out, though... | |
| Jul 31, 2017 at 9:45 | comment | added | AnoE | Just out of interest: while the answer is splendid and answers the question perfectly, also in a very didactic way, I wonder if there ever has been a thought process like this when people "back then" "invented" the concept of speed/velocity. I.e., if we look at the historic time interval between "there is no physical formula for speed" vs. "there is the well-known formula", was there a complicated process in between? Or was it just plain obvious to everyone, as soon as people had the concepts of time and distance as numeric factors? | |
| Jul 31, 2017 at 8:40 | comment | added | user | I don't have the reputation to make minor edits here, but "we must eb able to" should be "we must be able to". | |
| Jul 31, 2017 at 0:11 | comment | added | Monty Harder | I was taught that "speed" was a scalar, and "velocity" a vector. So if you're talking about the scalar "distance" as the "d" in the equation, then you'd better be talking about "speed" rather than "velocity", or you're doing it wrong. | |
| Jul 30, 2017 at 23:45 | comment | added | FGSUZ | @Pancake_Senpai okay I see you mean $(\Delta d)^2$ and not $\Delta(d^2)$. You are right on your thoughts, so you are not an idiot at all haha (maybe I am). Answering this has made me be thinking for a long time. At the moment the only thing I can think of is... just simplicity. Defining $d^2/t$ would be very complicated for non-integers, and given a linear space, we would quickly invent a new magnitude to be $d/t$. In fact we always search for linear sensors/instruments because of that... I don't know, maybe I can do better tomorrow haha. Very nice question indeed. | |
| Jul 30, 2017 at 23:04 | comment | added | Pancake_Senpai | @FGSUZ Reading your answer to this question has been very interesting, but I don't understand the bit about why $\Delta d^2$ is discarded. $\Delta d$ would be the same for 50->60 as it would be for 70->80, and therefore so would $\Delta d^2$. I appreciate that once the basic relationships (as in direct and inverse proportionalities) have been discovered they can be expanded upon (i.e. squares and the like can be determined) by experiment, but I don't think your explanation for negating the inverse square relation holds up. Please, if I'm being a idiot and am missing something then do tell me. | |
| Jul 30, 2017 at 21:42 | comment | added | FGSUZ | @JMac Yes, but I think we're talking about different things. Of course it is a convention, of course the name is random, and so on. What I was trying to say is that I believe everybody has a notion of velocity, and what I only did was tying to explain why we mathematically formalise that everyday concept like that, and I keep thinking that concept is acquired due to experience. Now we can wonder why time is (classically) linear, and why space is homogeneous. I think the concept is obvious only due to our experience. Maybe I'm not really understanding what you mean? Very interesting, though. | |
| Jul 30, 2017 at 21:32 | comment | added | JMac | @FGSUZ I just find this addresses a misconception. The fact is, the only "experience" that has to do with it is that we choose to say "velocity is a measure of distance per time" the same way we choose to define everything else. There's no everyday experience that makes us decide "yes, this we shall call speed!", it could have been called anything. When talking about speed you know more than just that we're talking about distance and time, we know that by definition we are talking about $v \equiv \frac d t$ it is equation we ourselves define. It's good it helped OP I guess though. | |
| Jul 30, 2017 at 20:53 | comment | added | FGSUZ | I'm so glad this was useful, as I don't use to know enough to help. @JMac That's a nice note. I guess you are right, it's true, I pre-assumed what $v$ is. After all, I think that the question didn't mean why we define a physical quantity like that, but "how and why our everyday experience yiedls that definition" . This is probably more philosophy but... I'm from the ones who think that space and time are innate ideas, and so its relation is acquired by experience. I think I only did a Socrates act: I only made explicity what was probably already inside our minds. Thanks again for your note | |
| Jul 30, 2017 at 20:48 | comment | added | JMac | This seems to pre-assume what velocity/speed is though. You say "Evidently (for a constant t) velocity increases if d does; and (for a constant space) v decreases if t rises. That constrains the ways we can define it. " But that already comes from the definition that speed is distance traveled during a set amount of time. | |
| Jul 30, 2017 at 20:48 | comment | added | dts | This is exactly what I was looking for! Thank you so much! | |
| Jul 30, 2017 at 20:43 | vote | accept | dts | ||
| Jul 30, 2017 at 20:41 | history | answered | FGSUZ | CC BY-SA 3.0 |