I do not see an actual question. Just Obamas presidential acceptance speech.
This sentence is basically saying that the large rock was competing in the race. In order for this not to happen, one could say, 'As Maura was competing in the race, a large rock that was sitting on the track caused her to trip and stumble a few steps.'
Hope this helped
A closed shop is one that hires only members of a union; an open shop's employees do not have to belong to a union.
Good luck!