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.
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.
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 
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.
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.
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.
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.
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.
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.
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).
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? 
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.
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…]
The usual term is “displacement”.
Ah, that does sound better. Good to know.
(And my [Da dit da doo dah dah] gets to still stand…)
Is that supposed to be Yakety Sax? Because that’s what I imagine playing while Bob’s circling you at great speed…