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An extremely important concept in the development of mechanics is that of momentum. The momentum of a body is defined as the product of its mass by its velocity. We shall use the symbol p to denote the momentum of a body. The momentum of a body is a vector quantity, for it is the product of mass, a scalar, by velocity, a vector. While momentum and kinetic energy are compounded of the same two ingredients, mass and velocity, they are quite different concepts, and one aspect of their difference may be seen in the fact that momentum is a vector while energy is a scalar quantity.