QA: “Speed” vs “velocity”

Each week here at the Australian Writers’ Centre, we dissect and discuss, contort and retort, ask and gasp at the English language and all its rules, regulations and ridiculousness. It’s a celebration of language, masquerading as a passive-aggressive whinge about words and weirdness. This week, there are speed bumps…

Q: Hi AWC, what’s the difference between “speed” and “velocity”?

A: Well, one will get you high as a kite and the other only high on Virgin airlines if you have enough points.

Q: You’re hilarious. But I’m talking about describing going fast – is there a difference in calling it your speed or your velocity?

A: Yes and no.

Q: You’re not very helpful today.

A: Allow us to explain. Imagine you’re on a bus with Keanu Reeves and Sandra Bullock and if the bus goes below 50 miles per hour, it will explode.

Q: Let me guess, that’s Speed?

A: Yes. Not only is it the 1994 action film Speed, BUT 50 miles per hour is ALSO the speed. It’s actually a word that dates all the way back to the 1200s – with its original meaning simply being “a rate of motion or progress”.

Q: Sure – it’s how FAST something is going, right?

A: Right. And scientifically, speed is the rate at which an object covers a specified distance.

Q: Speed = distance over time!

A: Precisely. So, pop quiz hotshot – if you travel 50 miles in one hour, your speed is 50 miles per hour. If however, you only travel 49 miles in that hour, what then?

Q: Um, your speed is 49 m–

A: No! Your bus explodes. Please, try to keep up.

Q: Oh, yeah that’s right. And this all makes perfect sense. But what about “velocity”? In the bus scenario, what is the velocity of the bus?

A: Now you’re asking the right questions. The velocity would ALSO be 50 miles per hour, but it would include a direction (e.g. NORTH). This is because velocity is the rate at which an object changes its position.

Q: Sounds pretty much the same as “the rate at which an object covers a specified distance.”

A: Well, not quite.

Q: I’d like to get off at the next stop please.

A: You see, speed is “scalar” – a simple sliding scale of magnitude. Faster or slower. But velocity is “vector-based” – meaning it’s concerned with magnitude AND direction. 

Q: My mother is often concerned about my lack of direction…

A: The word “velocity” came to English in the 1400s from the Latin velocitas for “swiftness; speed”. This meaning also gave us the “velocipede” in 1819.

Q: A very fast bug with lots of legs?

A: No, the velocipede was the ancestor of the modern bicycle (which would take another 30 years to arrive). The name lives on in “velodromes’ – indoor bicycle tracks. Even the “velociraptor” dinosaur was named as such when it was discovered in 1924, due to its supposed speed.

Q: So, speed and velocity can be basically the same value, but not always?

A: Not always. Pop quiz – if Sandra travels north 50 miles in one hour and then turns around and comes back south at the same speed to where she began, what is her average speed?

Q: Well, the bomb hasn’t gone off, so 50 miles per hour throughout?

A: That’s right. And her velocity?

Q: Ummmmm.

A:  Her velocity would be the same as the amount of chemistry between Sandra and Keanu in that movie. Zero.

Q: Really? Zero?

A: Really. Velocity is how much distance you have displaced since the start point – or the rate of change of position. So if you return to the start, you are in the same position. Zero change.

Q: No, I meant, did you REALLY think they had no chemistry? I thought they were quite cute together.

A: Okay, maybe a little. But the velocity would still be zero.

Q: So, when would you use “velocity”?

A: “Velocity” is used by scientists to calculate things where direction is crucial, like a rocket going to the moon or in defining “terminal velocity” – the maximum speed an object falls through the air..

Q: Wait, wasn’t Terminal Velocity ANOTHER 1994 movie?

A: Yes! It was about skydiving and starred Charlie Sheen and Nastassja Kinski.

Q: I guess that’s why they call them “MOTION pictures”, right?

A: Hilarious.

Q: So to recap, “speed” and “velocity” are both about rapidity of motion, but velocity cares about the direction and changing position while speed just wants to put its foot on the accelerator and do burnouts in the carpark.

A: Yeah. Speed is the simple metric that drivers, athletes and racing camels use, but you can get boffin-like and look at velocity of any speed metric too, if you know the displacement.

Q: And writers can probably use “speed” and “velocity” interchangeably when talking about simply going fast, yeah?

A: If it’s generic, dictionaries like Macquarie tend to keep both meanings simply about swiftness or rapidity of motion. Only under a “Physics” listing does it get more specific for each. It’s still a nice distinction to know.

Q: It is.  I’m curious – when did the concept of “speeding” first get used for vehicles?

A: Speed limits were first put in place for steam trains in 1879, and the idea of cars “speeding” was established by 1908. Meanwhile, speed humps and bumps were first introduced in 1975. New Zealand calls them “judder bars”.

Q: How onomatopoeic of them. 

A: Indeed.

Q: Oh, wait, what happened to the bomb on the bus?

A: It’s fine. That all happened years ago and they all went on to have successful Hollywood careers – all thanks to Speed!

Q: I actually think many Hollywood careers can thank speed…

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