A seat belt helps to prevent injury in the event of a car crash by reducing the velocity of a body as it experiences a sudden decrease in speed. Due to the body's inertia, which is its 'resistance to a change in speed or direction of travel', a passenger in a vehicle will want to continue travelling forwards once the car has reached a sudden stop. If the vehicle is travelling at 50mph and crashes into a brick wall, instantly reducing its velocity to zero, the passenger will continue moving forwards at 50mph unless there is something in front of them to create a 'stopping force'. This is because the velocities of the car and passenger are independent.. Hope this helps you!(: Good luck.