Aristotle describes two kinds of motion: "violent" or "unnatural motion", such as that of a thrown stone, in the "Physics" (254b10), and "natural motion", such as of a falling object, in "On the Heavens" (300a20). In violent motion, as soon as the agent stops causing it, the motion stops also: in other words, the natural state of an object is to be at rest, since Aristotle does not address friction. With this understanding, it can be observed that, as Aristotle stated, heavy objects (on the ground, say) require more force to make them move; and objects pushed with greater force move faster. This would imply the equation