# What's the difference between "speed" and "velocity"? **URL:** \ **Category:** Factual Questions\ **Created:** [July 29, 2010, 1:48am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403 "2010-07-29T01:48:20Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [July 29, 2010, 1:48am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/1 "2010-07-29T01:48:20Z")
I’ve seen in a couple of threads with physics and aerodynamics people, either talking to themselves, or explaining this or that to a layman, that when someone says “speed” someone else gently corrects and says the more accurate term is “velocity.” What’s the dif? ---
**Author:** ![notsoheavyd3](https://avatars.discourse-cdn.com/v4/letter/n/85e7bf/32.png) [@notsoheavyd3](https://boards.straightdope.com/u/notsoheavyd3)\ **Post date:** [July 29, 2010, 1:51am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/2 "2010-07-29T01:51:12Z")
> [@Leo\_Bloom](#): > > I’ve seen in a couple of threads with physics and aerodynamics people, either talking to themselves, or explaining this or that to a layman, that when someone says “speed” someone else gently corrects and says the more accurate term is “velocity.” > What’s the dif? Velocity is speed and a direction ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [July 29, 2010, 1:55am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/3 "2010-07-29T01:55:47Z")
> [@notsoheavyd3](#): > > > [@Leo\_Bloom](#): > > > > I’ve seen in a couple of threads with physics and aerodynamics people, either talking to themselves, or explaining this or that to a layman, that when someone says “speed” someone else gently corrects and says the more accurate term is “velocity.” > > What’s the dif? > > Velocity is speed and a direction That. You could also say that speed is a scalar and velocity is a vector. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [July 29, 2010, 2:06am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/4 "2010-07-29T02:06:29Z")
> [@notsoheavyd3](#): > > > [@Leo\_Bloom](#): > > > > I’ve seen in a couple of threads with physics and aerodynamics people, either talking to themselves, or explaining this or that to a layman, that when someone says “speed” someone else gently corrects and says the more accurate term is “velocity.” > > What’s the dif? > > Velocity is speed and a direction To expand on this. When I drive to work on one particular stretch of road. My speed is 40mph, my velocity is North at 40mph. ---
**Author:** ![GHO57](https://avatars.discourse-cdn.com/v4/letter/g/df788c/32.png) [@GHO57](https://boards.straightdope.com/u/GHO57)\ **Post date:** [July 29, 2010, 2:07am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/5 "2010-07-29T02:07:09Z")
Scalar vs. vector quantity. Meaning speed has a value, and velocity has a direction. speed = distance traveled/time traveled Velocity = change in position/time The difference (in everyday use) comes when you consider “change in position” Driving your car forward for a mile, and coming back, means your average velocity is zero, as there’s no change in position. Your speed has a non-zero average as you moved 2 miles in a certain time. Here, go study [Speed versus Velocity](http://www.physicsclassroom.com/class/1dkin/u1l1d.cfm) 🙂 ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [July 29, 2010, 2:21am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/6 "2010-07-29T02:21:35Z")
> [@GHO57](#): > > Scalar vs. vector quantity. > > Meaning speed has a value, and velocity has a direction. Speed has a quantity (what you called a value). Velocity has a quantity and a direction. Speed: I’m driving at 40 miles per hour. Velocity: I’m driving due north at 40 miles per hour. (That means my velocity in the east or west direction is 0 and my velocity in the south direction is -40 miles per hour). Let’s not get into the spherical nature of the earth, and just assume that the cardinal directions are in 2-D. ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [July 29, 2010, 3:28am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/7 "2010-07-29T03:28:12Z")
Well, the spherical nature of the Earth doesn’t really matter, since velocity of movement along a sphere at a point lives in the tangent plane to that point (well, actually, it’s a linear map from time to this tangent plane). And at any point other than the poles, the cardinal directions do indeed correspond to an orthogonal basis for the tangent plane. ---
**Author:** ![notsoheavyd3](https://avatars.discourse-cdn.com/v4/letter/n/85e7bf/32.png) [@notsoheavyd3](https://boards.straightdope.com/u/notsoheavyd3)\ **Post date:** [July 29, 2010, 3:54am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/8 "2010-07-29T03:54:39Z")
> [@friedo](#): > > That. You could also say that speed is a scalar and velocity is a vector. True but given that velocity is often the first vector they teach people about I figured he probably hadn’t be taught vectors and scalars yet. ---
**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 29, 2010, 3:58am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/9 "2010-07-29T03:58:43Z")
This also means that “velocity” isn’t always the technically-correct word to use. For instance, you’ve heard of escape velocity? That should actually be escape speed, since direction doesn’t matter for it. ---
**Author:** ![Nars\_Glinley](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nars_glinley/32/8177_2.png) [@Nars\_Glinley](https://boards.straightdope.com/u/Nars_Glinley)\ **Post date:** [July 29, 2010, 4:03am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/10 "2010-07-29T04:03:24Z")
This is probably a stupid question but if speed and velocity are different, why are they measured in the same units? ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [July 29, 2010, 4:09am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/11 "2010-07-29T04:09:26Z")
> [@Nars\_Glinley](#): > > This is probably a stupid question but if speed and velocity are different, why are they measured in the same units? They aren’t, really. “miles per hour” is a unit of speed; “miles per hour times a direction” is a ‘unit’ of velocity. [Though people perhaps don’t speak of “units” so much when discussing quantities which are more than one-dimensional, using terminology like “basis” instead] However, the _size_ of a velocity is a speed, which is perhaps where the confusion comes from, since so often, one mentions a velocity by way of implicitly referring to its size. ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [July 29, 2010, 4:17am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/12 "2010-07-29T04:17:16Z")
> [@notsoheavyd3](#): > > True but given that velocity is often the first vector they teach people about I figured he probably hadn’t be taught vectors and scalars yet. I think we do students a great disservice by not teaching them more about abstract linear algebra before or alongside all the many things we toss at them which it is ubiquitous throughout. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [July 29, 2010, 4:27am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/13 "2010-07-29T04:27:49Z")
> [@Nars\_Glinley](#): > > This is probably a stupid question but if speed and velocity are different, why are they measured in the same units? They use some of the same units, but the end unit isn’t the same. For example, speed can be described in terms of miles/hour and acceleration can be described in miles/hour/hour. They both use miles and hours to make up their units, but it’s not really the same. Hmmm, maybe that’s not quite the same. How about this. There is a piece of paper 5 feet from me VS there is a piece of paper 5 feet to my right. Velocity is a bit more descriptive then speed. Without a basic physics lesson that gets in to when and why you use which one you could probably think of it like that. (Actually, here’s a easy example. Remember the old math question from grade school. If train A leaves here at 5:00 going 40 MPH and Train B leaves here at 6:00 going 35 MPH… You use velocity to figure out when and where they’ll meet. You use speed to find out how far the conductor will fly when they smash into each other and he gets ejected). ---
**Author:** ![notsoheavyd3](https://avatars.discourse-cdn.com/v4/letter/n/85e7bf/32.png) [@notsoheavyd3](https://boards.straightdope.com/u/notsoheavyd3)\ **Post date:** [July 29, 2010, 4:35am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/14 "2010-07-29T04:35:16Z")
> [@Chronos](#): > > This also means that “velocity” isn’t always the technically-correct word to use. For instance, you’ve heard of escape velocity? That should actually be escape speed, since direction doesn’t matter for it. Sure it does Chronos. I mean if you try to go at escape speed straight into the ground you don’t get very far:) ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [July 29, 2010, 4:38am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/15 "2010-07-29T04:38:26Z")
> [@notsoheavyd3](#): > > Sure it does Chronos. I mean if you try to go at escape speed straight into the ground you don’t get very far:) Hey, why not make gravity work for you rather than against you? 🙂 ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [July 29, 2010, 5:53am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/16 "2010-07-29T05:53:42Z")
The difference between speed and velocity is important when you’re talking about acceleration. Acceleration is a change in velocity but not necessarily a change in speed. If an object is moving at 40 mph north and then turns to the east still travelling at 40 mph, its velocity has changed and it has experienced acceleration, but its speed has not changed. ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [July 29, 2010, 6:01am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/17 "2010-07-29T06:01:50Z")
Indeed. All this business about “speed” and “velocity” is just a reflection of the difference between “distance” and, uh, let’s call it “deviation (in position)”. If I tell you Bob is 1 meter away from me, I’ve told you his distance from me. If I tell you Bob is 1 meter north from me, I’ve told you his deviation from me, which involves direction as well as distance. And note that Bob can be moving at great speed without his distance from me ever changing; he could be circling me at whatever speed he likes, his deviation changing by virtue of its direction changing, but the distance remaining fixed. [Da dit da doo dah dah…] ---
**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 29, 2010, 6:06am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/18 "2010-07-29T06:06:37Z")
> [@](#): > > Indeed. All this business about “speed” and “velocity” is just a reflection of the difference between “distance” and, uh, let’s call it “deviation (in position)”. The usual term is “displacement”. ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [July 29, 2010, 6:08am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/19 "2010-07-29T06:08:20Z")
Ah, that does sound better. Good to know. (And my [Da dit da doo dah dah] gets to still stand…) ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [July 29, 2010, 7:54am UTC](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403/20 "2010-07-29T07:54:18Z")
> [@Indistinguishable](#): > > [Da dit da doo dah dah…] Is that supposed to be Yakety Sax? Because that’s what I imagine playing while Bob’s circling you at great speed… [Next page](https://boards.straightdope.com/t/whats-the-difference-between-speed-and-velocity/548403.md?page=2)