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There are two basic classes of forces, 38depending on whether the agent touches the object or not. Contact forces are forces that act on an object by touching it at a point of contact. The majority of forces are contact forces. 依據能動者是否接觸物體，力可以分為兩種。接觸力是透過碰觸物體而施展。大部分的力都是接觸力。A referring to 涉及；與...相關B depending on 依靠C looking into 調查、研究D tearing from 撕掉