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The vis insita, or innate force of matter is a power of resisting, by which every body, as much as in it lies, endeavors to preserve in its present state, whether it be of rest, or of moving uniformly forward in a right line. In common usage, however, people may also use the term "inertia" to refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), and sometimes its momentum, depending on context (e.g. "this object has a lot of inertia"). The term "inertia" is more properly understood as a shorthand for "the principle of inertia" as described by Newton in Newton's First Law of Motion which, expressed simply, says "An object that is not subject to any outside forces moves at a constant velocity, covering equal distances in equal times along a straight-line path." In even simpler terms, inertia means "A body in motion tends to remain in motion, a body at rest tends to remain at rest." On the surface of the Earth the nature of inertia is often masked by the effects of friction which brings moving objects to rest relatively quickly unless they are coasting on wheels, well lubricated, or perhaps falling or going downhill (and thus being accelerated by gravity). This is what misled classical theorists such as Aristotle who believed objects moved only so long as force was being applied to them.[1] Several Muslim scientists from the medieval Islamic world wrote Arabic treatises on theories of motion. In the early 11th century, the Islamic scientist Ibn al-Haytham (Arabic ??? ??????) (Latinized as Alhacen) hypothesized that an object will move perpetually unless a force causes it to stop or change direction. Alhacen's model of motion thus bears resemblance to the law of inertia (now known as Newton's first law of motion) later stated by Galileo Galilei in the 16th century.[2] Alhacen's contemporary, the Persian scientist Ibn Sina (Latinized as Avicenna) developed an elaborate theory of motion, in which he made a distinction between the inclination and force of a projectile, and concluded that motion was a result of an inclination (mayl) transferred to the projectile by the thrower, and that projectile motion in a vacuum would not cease.[3] He viewed inclination as a permanent force whose effect is dissipated by external forces such as air resistance.[4] [3] Avicenna also referred to mayl to as being proportional to weight times velocity, which was similar to Newton's theory of momentum.[5] Avicenna's concept of mayl was later used in Jean Buridan's theory of impetus.
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