There are a couple of things to think about with your question.

1. People who don't wear seat belts are much more likely to be ejected from their seats in a car crash! This is because as the car is moving, it has a certain amount of inertia, or tendency to remain in motion. When the car stops suddenly, a passenger's body will tend to keep moving forward, because of inertia. If the passenger is not wearing a seatbelt, he or she could hit the dashboard or even be ejected through the front windshield.

2. Another thing to keep in mind is that seatbelts are "stretchy." When a car suddenly stops, the seatbelt increases the time over which the passenger's body comes to a stop. This in turn decreases the force the passenger feels when the car stops, and decreasing the chance of severe injury.

The concept is the rate at which passenger's body changes speed, and thus the amount of force that the passenger's body is subjected to. The shorter the time over which the passenger is forced to stop moving, the more force is applied to his body to make him stop. Apply enough force, and it causes damage.

Wearing a seatbelt slows down the passenger's motion by causing the passenger to start slowing down before he or she is thrown into the dashboard when the car suddenly stops. This means that the passenger has more time in which to stop moving, and thus is less likely to break bones or suffer other physical injury.