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Inertia is an object'??s resistance to a change in its motion. This could involve a change of speed or direction, an attempt to move a stationary object or an attempt to stop an object that is already moving. The idea is linked to Isaac Newton's first law of motion, which states that an object's motion will not change unless a force acts on it. Inertia depends on mass, since the more massive an object is, the more it resists a change in motion.