The minimum number of objects required for a force to be present is two, as established by Newton's Third Law, which states that forces come in pairs. In a physical system, such as a person pushing a block, the block is the object of interest, while the person is not included in the system. A free body diagram illustrates this concept by depicting the block and the force vector applied to it, confirming that for every action, there is an equal and opposite reaction.
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kuruman said:You are correct. The number is 2. Newton's 3rd Law says that forces come in pairs, so if you have a single force acting on an object (as in a free body diagram), you can safely deduce that there is another object (somewhere in the Universe and outside you free body diagram) that experiences a force of equal magnitude but in the opposite direction.