Sentence:  The `` moving '' picture of the train or the wave coming at the audience is , to be sure , more intense than a still picture of the same subject , but the difference is really one of degree ; the cinematic <WSD>element</WSD> of time is merely used to increase the realism of an object which would still be reasonably realistic in a still photo . 
Meaning of the word marked with <WSD> tags:  an artifact that is one of the individual parts of which a composite entity is made up; especially a part that can be separated from or attached to a system