If the system is isolated, then and the total momentum remains the same both in magnitude and in direction at all times during the motion. Note also that, when applying the above equation, you consider external forces acting on different objects inside your system. This is unlike in Newton's second law, where you only sum up forces acting on the same object. As we mentioned before, it is impossible to accurately describe the complex time-dependent forces exerted e.g. during a collision. In this Section, we discuss a method of estimating them.
Lets go back to the question of how to stop a moving object.
If we exert a small force for a long period of time, we can achieve the same result as with a much larger force exerted over a much shorter period of time. For example, you can stop your car either by stepping on the brake real hard or by pressing the brake softly for a long period of time. Therefore, the change in momentum produced by a certain force must depend both on the force magnitude and the time of action. We can obtain such a dependence from Newton's law expressed in the form . It is the impulse of a force that produces momentum changes.