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In physics, a force is a push or pull between objects. It is called an interaction because if one object acts on another, its action is matched by a reaction from the other object. This idea is known as Newton's third law, where action and reaction are "equal and opposite" (matched).The objects are just the things the force acts between. Different forces act between different sorts of object. For example, gravity acts between objects with mass, like the sun and the earth. Another example is electromagnetic force, which acts between objects with charge, like an electron and the nucleus of an atom. Gravity and electromagnetic force are two examples of forces.