Photos taken with a camera do not represent a single moment of time. Due to technological constraints these shots stand for some scene over a brief period of time. This time frame depends on the camera's shutter speed. In motion blur, any object moving with respect to the camera will look blurred or smeared along the direction of relative motion.
A bullet being shot through an apple, an egg being cracked into a fan, or a play-by-play of Pancho Gonzales's famous serve ... MIT professor Harold Edgerton invented the strobe flash in the 1930s – and his stroboscopic photography captured amazing moments that would otherwise be missed in the blink of an eye