why we always consider a level surface to prove that gradient is perpendicular to tangent vector to the surface...  what if the particle moves on a surface which doesn't has a  constant value throughout  its domain of definition####[['level surface', 'Mathematical_Related_Concept', 'NEU'], ['gradient', 'Mathematical_Related_Concept', 'NEU'], ['tangent vector', 'Mathematical_Related_Concept', 'NEU'], ['constant value', 'Mathematical_Related_Concept', 'NEU'], ['domain of definition', 'Mathematical_Related_Concept', 'NEU'], ['perpendicular', 'Mathematical_Related_Concept', 'NEU']]