**Momentum** is a property of any moving object. For a slow moving object it is given by the mass times the velocity of the object. For an object moving at close to the speed of light this definition gets modified. The total **momentum** is a conserved quantity in any process. Physicists use the letter p to represent **momentum**, presumably because m was already used for mass, n for number, and o is too much like zero. (source unknown)

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