Juanita studies several free-body diagrams in which a normal force is shown. What is always true about a normal force in a free-body diagram?

A normal force is the support force exerted by a surface on an object, and it always acts perpendicular to that surface. It is not necessarily opposite the net force or the object's weight; those directions depend on the situation. Therefore, the correct choice is “It is perpendicular to a surface.”

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