A ball is dropped in front of a meter stick and lit by a strobe light. A long exposure photograph captures the position of the ball at each evenly spaced flash of light. The acceleration of the ball can then be measured from the photo.  Note that the frame rate of the video capture (30fps) is quite close to the strobe rate (15Hz). This is why the strobe flashes in the slow motion video don’t appear to be exactly evenly timed. See the final image on Flickr.