Great question! Speed is how fast something is going. Another way to think of this is as how far you can go in a certain amount of time. For example, if you’re walking slowly, your speed is probably about 2 miles per hour. This means that in one hour, you’ll be able to move 2 miles. If you’re running quickly, your speed is probably closer to 7 or 8 miles per hour.
If you’ve ever looked at the speedometer of a car, you’ve seen that it measures speed in mph (or kph). MPH is short for miles per hour (and kph is short for kilometers per hour). So if your car says your speed is 50 mph, then you can go 50 miles in one hour if you keep going at that speed.
The word "velocity" means almost the same thing as speed, except that velocity also means which way you’re going. For example, if you’re running in a circle, then your velocity is constantly changing, because your direction is changing. But your speed doesn’t change at all unless you start running faster or slower.
(republished on 07/11/06)