Law of Conservation of Momentum What is Momentum? It is inertia in motion. It is a measure of the quantity of motion, defined as the product of the mass and the velocity of a body, and determining the length of time during which constant force must act on a moving body to bring it to rest. It is a vector quantity, meaning it has the same direction as the velocity of the object. It is the force built up by a moving body. Momentum is represented by p. The equation for momentum then is p = mv where p is the momentum of the body, m is the mass of the body, and v is the velocity. The SI unit of momentum is kilogram-meter per second (kg . m/s). Consider this: Josef drove his car from 25 kph to 35 kph. Would the momentum of Josef's car change with the change in velocity? If a friend twice as heavy as you are rides with you on your scooter, would the momentum of the scooter also change? Yes, the momentum of the car and of the scooter will change. What does this say about the relationship among mass, velocity, and momentum? The momentum of an object depends on its mass and its velocity. A change in velocity, in mass, or in both means a change in momentum. In most cases, though, it is the velocity that changes. Remember that when the velocity changes, the object accelerates. When there is acceleration, a net force is exerted on the object.