# Why do we use velocity for flow rates, not speed? **URL:** \ **Category:** Factual Questions\ **Created:** [July 15, 2011, 10:02am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113 "2011-07-15T10:02:45Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 10:02am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/1 "2011-07-15T10:02:45Z")
Hi Dopers, As I understand it, telling somebody that they have a flow rate of, say 30m/s, is giving them the speed of the material in the pipe, not the velocity as no vector or direction has been defined. Is that right? Is there some kind of assumption that means that the vector is ‘down the pipe’? Is speed _technically_ the correct measurement? As I say, my understanding is that 30m/s is scalar, and cannot be used to measure velocity as there is no direction/vector. Should the term ‘relative velocity’ as in ‘realtive to the pipe’ be used as it is more accurate? Bored at work? Yes, why do you ask? 🙂 ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 15, 2011, 10:13am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/2 "2011-07-15T10:13:35Z")
> [@L11CFR](#): > > Hi Dopers, > > As I understand it, telling somebody that they have a flow rate of, say 30m/s, is giving them the speed of the material in the pipe, not the velocity as no vector or direction has been defined. > > Is that right? Is there some kind of assumption that means that the vector is ‘down the pipe’? > > Is speed _technically_ the correct measurement? > > As I say, my understanding is that 30m/s is scalar, and cannot be used to measure velocity as there is no direction/vector. > > Should the term ‘relative velocity’ as in ‘realtive to the pipe’ be used as it is more accurate? > > Bored at work? > > Yes, why do you ask? > > 🙂 I’m not expert on pipes, but my guess would be is that it’s the mean velocity in the direction of the flow. It’s not wrong to call it velocity as the direction is very much implied. ---
**Author:** ![mac\_bolan00](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@mac\_bolan00](https://boards.straightdope.com/u/mac_bolan00)\ **Post date:** [July 15, 2011, 10:18am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/3 "2011-07-15T10:18:03Z")
in the mines we always used gallons/minute. ---
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 10:22am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/4 "2011-07-15T10:22:57Z")
> [@These are my own pants](#): > > It’s not wrong to call it velocity as the direction is very much implied. I picked pipes as that is what I am working on at the moment. Or trying to 🙂 I was talking with a co-worker and we got chatting about and couldn’t decide whether it was just an accepted terminology, albeit _technically_ incorrect, or that you could, despite the protestations of the few websites we looked at, define a velocity without direction, only implication. You also raise a good point. Technically, from a pedant’s point of view, is it mean velocity as opposed to plain ole vanilla velocity. Or relative velocity as I mentioned in the OP. ---
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 10:29am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/5 "2011-07-15T10:29:52Z")
Missed edit window ☹ Maybe instantaneous velocity should be used instead? Are velocity and instantaneous velocity interchangeable? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 15, 2011, 10:33am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/6 "2011-07-15T10:33:56Z")
> [@L11CFR](#): > > I picked pipes as that is what I am working on at the moment. Or trying to 🙂 > > I was talking with a co-worker and we got chatting about and couldn’t decide whether it was just an accepted terminology, albeit _technically_ incorrect, or that you could, despite the protestations of the few websites we looked at, define a velocity without direction, only implication. > > You also raise a good point. Technically, from a pedant’s point of view, is it mean velocity as opposed to plain ole vanilla velocity. Or relative velocity as I mentioned in the OP. Yep it’s perfectly acceptable. For example usually there’s no need to say which direction escape velocity is in as it’s implicit from the definition of escape velocity that will be radially outwards from the centre of mass. ---
**Author:** ![mac\_bolan00](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@mac\_bolan00](https://boards.straightdope.com/u/mac_bolan00)\ **Post date:** [July 15, 2011, 10:44am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/7 "2011-07-15T10:44:41Z")
> [@L11CFR](#): > > Missed edit window ☹ > > Maybe instantaneous velocity should be used instead? Are velocity and instantaneous velocity interchangeable? instantaneous velocity is a single measurement along a velocity curve (meaning there is acceleration/deceleration.) ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 15, 2011, 10:54am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/8 "2011-07-15T10:54:45Z")
> [@L11CFR](#): > > Missed edit window ☹ > > Maybe instantaneous velocity should be used instead? Are velocity and instantaneous velocity interchangeable? I would say instantaneous velocity is the most ‘correct’ defintion of velocity as velcoity itself is defined by d **x** /dt. In this case I don’t think the velocity being measured is really instantaneous though. When you work in curved spacetimes the distinction between vectors and vectors which are actually equivalence classes of spatially/temporally seperate vectors becomes very important. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [July 15, 2011, 11:15am UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/9 "2011-07-15T11:15:44Z")
> [@mac\_bolan00](#): > > in the mines we always used gallons/minute. Yeah, I was going to say, in practice flow is usually in volumetric or mass rates (l/s when I worked the mines, or M[sup]3[/sup]/s) not velocity. Measuring which [is not as trivial](http://en.wikipedia.org/wiki/Mass_flow_meter) as tossing a cork in a stream and pacing it, BTW. Unless you’re doing fluid dynamics, like hydrology, in which case you \*do \*want the velocity, but it’d be spot velocity or averaged velocity profile, 'cos of wall friction, turbulence etc. IOW, it’s incorrect to say that fluid flow in a pipe has “a” velocity. ---
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 12:07pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/10 "2011-07-15T12:07:42Z")
> [@MrDibble](#): > > Yeah, I was going to say, in practice flow is usually in volumetric or mass rates (l/s when I worked the mines, or M[sup]3[/sup]/s) not velocity. Measuring which [is not as trivial](http://en.wikipedia.org/wiki/Mass_flow_meter) as tossing a cork in a stream and pacing it, BTW. I’m referring to steam in this instance. You are perfectly correct in that we measure flow rate as (mass/volume)/time but always at a specified velocity and pressure for pipe sizing. For instance we would say “At 14barg with a velocity of 30m/s you have a flowrate of 7388kg/hr in a DN100 pipe” Should have tried to be more specific on the OP - so sorry for any confusion :smack: > [@These are my own pants](#): > > For example usually there’s no need to say which direction escape velocity is in as it’s implicit from the definition of escape velocity that will be radially outwards from the centre of mass. But the clue is in the title there 🙂 In my example above, is velocity correct because of the implication that the flow rate is ‘down the pipe in the direction of flow’ or is speed, again technically, the more accurate term to use? ---
**Author:** ![Arjuna34](https://avatars.discourse-cdn.com/v4/letter/a/eb8c5e/32.png) [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)\ **Post date:** [July 15, 2011, 12:32pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/11 "2011-07-15T12:32:19Z")
> [@L11CFR](#): > > As I say, my understanding is that 30m/s is scalar, and cannot be used to measure velocity as there is no direction/vector. In one sense, there is a direction - the scalar value can be positive or negative, with one being defined as “up the pipe”, and the other being “down the pipe”. This is analogous with electrical current in a wire, where positive or negative current represents electron flow in one of two directions through a wire, irrespective of the physical orientation of the wire with respect to the outside world. Of course, this is a simplification, ignoring any gradients inside the pipe/wire, etc. and just treating the flow as a uniform effect. But that simplification is useful much of the time. ---
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 12:50pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/12 "2011-07-15T12:50:48Z")
> [@Arjuna34](#): > > Of course, this is a simplification, ignoring any gradients inside the pipe/wire, etc. and just treating the flow as a uniform effect. But that simplification is useful much of the time. Indeed and I completely understand how it is useful and I realise that the term velocity in my example is completely understandable. To all intents and purposes it makes no odds to anyone save some bored people here in my office. My question came about because piping is not always straight and someone suggested that if you had a piece of pipe going 3m North, 4m West and the steam made it from one end to the other in 1 second, the steam speed would be 7m/s and the mean velocity would be 5m/s NorthWest. As I say, if you said the steam velocity was 7m/s everybody would understand what you meant. What I was trying to ask in the first instance, obviously badly :), was would velocity be the most accurate technically pendant-pleasing way of describing it, since the vecotr is only implied as being ‘down the pipe’ and would, if that is not the case, speed be the more technically pedant-pleasing way to describe it. Is that any clearer - I’m pretty bad at this 🙂 ---
**Author:** ![vd](https://avatars.discourse-cdn.com/v4/letter/v/b19c9b/32.png) [@vd](https://boards.straightdope.com/u/vd)\ **Post date:** [July 15, 2011, 1:14pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/13 "2011-07-15T13:14:29Z")
You are basically modeling the position of a bit of steam in the pipe in one dimension. Even with a curved pipe, x can be the distance down the pipe. So dx/dt is a one dimensional vector. v = ( speed ) ---
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 1:16pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/14 "2011-07-15T13:16:35Z")
> [@vd](#): > > You are basically modeling the position of a bit of steam in the pipe in one dimension. Even with a curved pipe, x can be the distance down the pipe. So dx/dt is a one dimensional vector. > > v = ( speed ) So they are equally correct? Cool - thank you! Now off to wondering why they would use velocity. Judging by some of the people here - I’m going to guess it is because it sounds more intelligenter 😉 ---
**Author:** ![BobLibDem](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/boblibdem/32/3149_2.png) [@BobLibDem](https://boards.straightdope.com/u/BobLibDem)\ **Post date:** [July 15, 2011, 1:19pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/15 "2011-07-15T13:19:23Z")
I think downstream is implied. Suppose your pipe has a bend in it. Then you’d have to specify which part of the pipe you’re talking about to make a vector out of it. It seems needlessly complicated to do so. ---
**Author:** ![L11CFR](https://avatars.discourse-cdn.com/v4/letter/l/9f8e36/32.png) [@L11CFR](https://boards.straightdope.com/u/L11CFR)\ **Post date:** [July 15, 2011, 1:31pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/16 "2011-07-15T13:31:22Z")
> [@BobLibDem](#): > > I think downstream is implied. I agree - I suppose the question could have been re-written to ask “If you specify a velocity with only an implied vector, is speed more technically accurate measurement?” Speed also simplifies bends etc as you rightly mention. But, from what my tiny and rapidly-approaching-the-weekend brain can grasp, **vd** nails it as displacement = distance as it is effectively one dimensional with the dimension being ‘along the pipe’. At least, that is how I read it. ---
**Author:** ![vd](https://avatars.discourse-cdn.com/v4/letter/v/b19c9b/32.png) [@vd](https://boards.straightdope.com/u/vd)\ **Post date:** [July 15, 2011, 1:46pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/17 "2011-07-15T13:46:58Z")
I can’t think of any reason why velocity would be a better term to use than speed. I was just trying to come up with a way to make it justifiable. Maybe it is just that “velocity” feels more engineery than “speed”. ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [July 15, 2011, 1:50pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/18 "2011-07-15T13:50:11Z")
> [@MrDibble](#): > > Yeah, I was going to say, in practice flow is usually in volumetric or mass rates (l/s when I worked the mines, or M[sup]3[/sup]/s) not velocity. Measuring which [is not as trivial](http://en.wikipedia.org/wiki/Mass_flow_meter) as tossing a cork in a stream and pacing it, BTW. > > Unless you’re doing fluid dynamics, like hydrology, in which case you \*do \*want the velocity, but it’d be spot velocity or averaged velocity profile, 'cos of wall friction, turbulence etc. IOW, it’s incorrect to say that fluid flow in a pipe has “a” velocity. Agreed. The OP is a bit confusing because you say “flow rate” then “velocity” which, to my ears, is wrong. When we talk about flow rates we are talking about conveyance - volume per time period - which is not the same as velocity. You can, however, take your flow rate and divide it by the pipe’s cross sectional area to get an average velocity. Q = VA. In the case of equations for velocity in your case, the direction of flow doesn’t matter. The fluid flows from a higher head to a lower head, no matter which direction that is. 🙂 ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 15, 2011, 2:15pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/19 "2011-07-15T14:15:56Z")
> [@L11CFR](#): > > I agree - I suppose the question could have been re-written to ask > > “If you specify a velocity with only an implied vector, is speed more technically accurate measurement?” > > Speed also simplifies bends etc as you rightly mention. > > But, from what my tiny and rapidly-approaching-the-weekend brain can grasp, **vd** nails it as displacement = distance as it is effectively one dimensional with the dimension being ‘along the pipe’. > > At least, that is how I read it. Now you’re on to it. It is treated as a one-dimensional problem. In that one dimensional space, vector & scalar are effectively synonymous concepts. In other words, you have magnitude and sign, but you don’t have direction. Direction is a meaningless concept in a 1-dimensional situation. The sign on the scalar speed value completely covers the legitimate difference between upstream vs downstream. Your earlier example of a pipe which goes North & then West and you tried to talk about the shorter distance & hence faster rate measured across the diagoanal is an example of meaninglessness; by jumping outside the available dimensions you’ve made nonsense. How far apart are New York & Sydney in the 8th dimension? Who knows, and for our purposes, who cares; we can’t go that way. Same thing applies to your North then West pipe. So \*this \*pedant holds that you’ve got a scalar speed in a 1-dimensional problem; calling it a vector velocity just invites making silly mistakes like trying to cut the North-to-West corner. And of course all of this ignores the reality that flow isn’t truly one-dimensional; there’s turbulence, boundary layers, etc. But at the bulk level flow can be usefully modeled as one-dimensional. And once you \*decide \*to use that model, to remain \*consistent \*with your decision, you have to use scalar speed, not vector velocity as your measure of **d** x/ **d** t. In a fully 4-D simulation including turbulence, corner effects, cross-section effects, etc., \*then \*it becomes meaningful to talk about vector velocities of individual parcels of fluid. At that point you also need a time parameter on each parcel vector. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 15, 2011, 5:42pm UTC](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113/20 "2011-07-15T17:42:57Z")
Quoth **These are my own pants** : > [@](#): > > Yep it’s perfectly acceptable. For example usually there’s no need to say which direction escape velocity is in as it’s implicit from the definition of escape velocity that will be radially outwards from the centre of mass. With escape speed there’s no need to say which direction it is, at all. Any direction at all will work just fine, so long as you don’t run into too much matter first. [Next page](https://boards.straightdope.com/t/why-do-we-use-velocity-for-flow-rates-not-speed/589113.md?page=2)