There are many factors concerning Newton's three laws of motion and seat belt use when a vehicle comes to a stop suddenly due to an accident. The first deals with inertia. According to HowStuffWorks, Newton put it this way: "Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it."
While a passenger often does not realize it, his body is simply an object in motion when traveling in a vehicle. It continues in motion until it is compelled to stop, even when the vehicle stops suddenly during the crash. The seat belt spreads the force across the torso and waist during this stop.
Newton's second law states "Force is equal to the change in momentum per change in time." This translates to F=MA, where M is the passenger's body weight and A is the acceleration. In the sudden stop of a vehicle, its acceleration is the speed it is traveling minus zero, the speed when stopped. This fact points to the importance of following speed limit laws.
Newton's third law states "For every action, there is an equal and opposite reaction." The force created by the second law is the same force required to stop the passenger's body. While passengers sometimes suffer seat belt injuries, they are typically less severe than potential head injuries.