When an object changes its velocity instantaneously, without any acceleration, the momentum of the object remains constant. This principle is known as the conservation of momentum.
According to Newton's second law of motion, the rate of change of momentum of an object is equal to the net force acting on it. If there is no net force acting on the object, the momentum remains constant.
In this scenario, the object experiences an instantaneous change in velocity, meaning that it goes from one velocity to another without any time for acceleration. Since acceleration is the rate of change of velocity over time, if there is no change in velocity over time, there is no acceleration.
As a result, the object's momentum before and after the instantaneous change in velocity remains the same. The magnitude and direction of the momentum vector are preserved, as long as no external forces act on the object during the process.