A body at rest tends to stay at rest. A body in motion tends to stay in motion. You are driving your car down the highway at 55 mph with your seat belt on. Suddenly, a dog crosses your path and you slam on the brakes. The car has forward momentum and were it not for your seat belt restraining you the forward momentum would propel you through the windshield. Of course, the speed and velocity of the vehicle is important in this scenario. At slower speeds the force, momentum, and velocity would be greatly reduced thus diminishing the need for a restraining device like a seat belt. Seat belts save lives!