为什么我们总是考虑水平表面来证明梯度与切向量表面垂直...如果粒子在表面上移动，而该表面在其整个定义域中都没有常数值，会怎样？####[['水平表面', 'Mathematical_Related_Concept', 'NEU'], ['梯度', 'Mathematical_Related_Concept', 'NEU'], ['切向量', 'Mathematical_Related_Concept', 'NEU'], ['常数值', 'Mathematical_Related_Concept', 'NEU'], ['定义域', 'Mathematical_Related_Concept', 'NEU'], ['垂直', 'Mathematical_Related_Concept', 'NEU']]