Working with a still medium, when it comes to saying “movement” in our images, there are challenges. When trying to “freeze” motion and get a sharp image of something moving, we pan so the film plane keeps pace with the subject. The element in the frame that is not moving physically and relatively is the background so that’s what we blur to say movement. And when there are elements in the frame moving perpendicular to the film plane, they will blur as well if we use a slow enough shutter speed to so that motion. This is a lot to think about when photographing a passing bicyclist or car. It gets even more complicated as you get to larger subjects and you want to say speed!
Here’s an example of what I’m talking about. This gorgeous Super Cub, it’s not thought of as a speed demon but it is going at least 90mph to fly. Now we can say visually it’s moving by simply using a slower shutter speed and point the plane towards the sun. The prop will blur and from that blur, the mind takes this still image and gives it movement. But it doesn’t say, speed. Both images were taken exactly the same, D4s / 80-400VR3 (Vulture Strap a must!) at 1/60. The only difference in taking the photo is the top one, I was hanging out of the plane and shooting down, the other I was sitting back inside and shooting across to the cub. But if you look at the two photos, one looks like the Cub is a bat out of hell and the other, like it’s just popping along. The difference? The one with the bat out of hell feel, the Cub is MUCH closer to the background, earth! Both were taken with the same f/stop and while on earth and shooting down here, DoF can radically change how much is in or out of focus the background might be. Not the case once you go up and you are for all intents and purposes focusing at infinity because infinity has no DoF. So to create the illusion of speed, the subject has to be close to the background. I encourage you to do your own experiment. Get a bicycle rider to ride their bike close to a wall and then 20 feet away. Watch what visually happens visually in your photos, you’ll soon see the speed. It’s a simple technique you might want to tuck in the back of your mind for the next time you want to say speed.