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Momentum is based on an object's mass and velocity. It describes the “motion content” of an object. Or, to put it another way: how much stuff is moving and how fast that stuff is moving. When momentum changes, it is due to an impulse. Impulses describe force with respect to time. Impulses happen in collisions (when two objects hit each other). The objects apply a force on each other over a given time, and the momentum of the objects change because of it. Newton's second law of motion defines force as the product of mass times acceleration. Since acceleration is the change in velocity divided by time, you can connect the two concepts with the following relationship: F • t = m • Δ v.