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.
I’m Not Victor...I’m Vector!
If you look at how steep the line on your distance-time graph is (technically called the slope), you can figure out your velocity.
In science, velocity is the rate at which something’s position changes. In our walking example, the fact that you had a speed of 50mph tells us how fast you were going, but it doesn’t tell us anything about your position. Were you walking in a straight line, zig-zagging down the street, or going in circles? All of those scenarios have the same speed, but different velocities.
While speed is measured by distance traveled in a given amount of time, velocity is the displacement of something in a given amount of time. Velocity is what scientists call a vector quantity, which is just a fancy name for a measurement that includes both a number (or magnitude) and a direction.
So if you walked 10 miles north, and it took you an hour to do so, your average speed would be 10 miles per hour while your average velocity would be 10 miles North per hour.
Keep in mind that while speed measures how far you’ve traveled (your distance) in a given amount of time, velocity measures how far you’ve traveled from where you started (your displacement) in a given amount of time.
This seems like a subtle distinction, but it’s an important one. If on your walk you went north for half the time, then turned around and walked south the other half the time, your average speed wouldn’t change (assuming you walked the same speed both ways), however your average velocity for the trip would be zero. That’s because since your final position was the same as your starting position, your displacement was 0.