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In classical mechanics, momentum (pl. momenta; SI unit kg·m/s, or, equivalently, N·s) is the product of the mass and velocity of an object (p = mv). For more accurate measures of momentum, see the section "modern definitions of momentum" on this page. It is sometimes referred to as linear momentum to distinguish it from the related subject of angular momentum. Linear momentum is a vector quantity, since it has a direction as well as a magnitude. Angular momentum is a pseudovector quantity because it gains an additional sign flip under an improper rotation. The total momentum of any group of objects remains the same unless outside forces act on the objects. Momentum is a conserved quantity, meaning that the total momentum of any closed system (one not affected by external forces) cannot change. The word for the general concept of momentum was used in the Roman Republic primarily to mean "a movement, motion (as an indwelling force ...)." A fish was able to change velocity (velocitas) through the momentum of its tail.[1] The word is formed by an accretion of suffices on the stem of Latin movere, "to move." A movi-men- is the result of the movere just as frag-men- is the result of frangere, "to break." Extension by -to- obtains movimentum and fragmentum, the former contracting to momentum.[2] The momentum was not merely the motion, which was motus, but was the power residing in a moving object, captured by today's mathematical definitions. A motus, "movement", was a stage in any sort of change,[3] while velocitas, "swiftness", captured only speed. The Romans, due to limitations inherent in the Roman numeral system,[clarify] were unable to go further with the perception.[citation needed]
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