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No, it is an article (indefinite article), which is a form of "determiner" that determines if a noun is a specific example or any one of the examples. "An" obstacle = any obstacle "The" obstacle = a certain obstacle
To impede.
There is no such thing as exactly vertical because either it is vertical or it is not. You cannot have approximately vertical - it is not vertical, then. Vertical means at 90 degrees to the horizon (or horizontal).
by moving vertical
vertical is from side to side