Speed and velocity are sometimes used differently in real life than they are in science. Ask Science discusses the differences and similarities between speed and velocity.
A while back, Grammar Girl and I discussed a few words that tend to mean something different to scientists than they do to most people. Today I want to discuss another group of terms that most people use in a way that’s different than their scientific meanings: speed and velocity..
The Need for Speed
Speed is a measurement of how far you go in a given length of time. Miles per hour or meters per second are both measurements of speed. Notice how both of those measurements are in the form “distance” per “time.” Anytime you see that form, you’re probably talking about speed.
There are two ways to think about speed: average speed and instantaneous speed. To figure out your average speed, you take how far you’ve traveled and divide it by how long it took you to do it. So if you walked 50 miles today and it took you just 5 hours to do so, you could say that your average speed was 50 miles per 5 hours.
Of course that’s a bit weird to say, so most people would simplify things by dividing both parts by 5 so that they could see the average distance traveled each hour. In this case you’d get a speed of 10 miles per hour.
Instantaneous speed tells your speed is at a particular moment of time. If you’re in a car, you can figure out your instantaneous speed pretty easily by just looking at the speedometer (speedometer literally means “speed measurer”). If the speedometer says 50 miles per hour, then your instantaneous speed at the exact moment you looked at it was (not surprisingly) 50 miles per hour.
If you don’t have a speedometer handy, then you can technically still figure out your instantaneous speed using calculus, but it becomes a bit trickier.
The Not-so-Handy Distance-Time Graph
As you might imagine, lots of things can impact your average speed. Let’s say for example that during that rather impressive 50 mile walk you went on earlier, you decided to stop after the second hour to take a little breather. As a result, during that second hour you only made it about 8 miles.
Just like one bad test score can pull down your average grade, covering less distance during part of your travels will lower your average speed.
One thing you can do to visualize these changes in speed is to use a distance-time graph. This is a graph where the distance traveled is plotted along the vertical axis while the time passed since starting your journey is plotting along the horizontal axis.
If you plot your distance every hour using this graph, you can draw a line between the points to see a visual record of how your speed changed over time.