In: Physics
The term “long exposure” used here is quite simple; it’s when the shutter is open for long time (more than a half of second could be already considered a long exposure). Because of such long exposure the photographer could introduce various effects into the photograph. For example, unsteady hands will produce blurry images and weird lines of bright lights in the picture. Examples of intentional application are panning, zooming and freezing the moment with flash. Some of the effects actually allow the photographer to “remove” moving objects from the picture.
So, let's get back on track - back to the idea of “dragging the shutter”. The idea is to capture the movement and express this movement in the photograph. There are two ends of this stick. First is that the camera stays still and the subject is moving. Or that the subject stays still and the camera moves. And in the middle is all kind of combinations of it. Such that you follow the moving subject with the camera; and the background is blurred.
In this way long exposure time affect the image of a moving projectile.