It turns out that Σ Fnet = ma is a special case of Newton's second law. Newton determined that a net force was something that caused a time rate of change of momentum, Δp/Δt or dp/dt, where momentum is defined as p = mv. The two descriptions are the same if the mass of the object in question does not change. Therefore, if there is no net force acting on an object or a system of objects, the momentum does not change. This statement is called conservation of momentum. Conservation of momentum, along with conservation of energy, is used in analyzing collisions between objects.