How does not wearing a seat belt affect the motion of crasch test dummy?

When a car comes to a sudden stop (such as in an accident), a passenger or driver who _is not wearing a seat belt_ keeps moving along with the same velocity as the car just before the sudden stop. However, the car and the objects inside it, such as the dashboard and windshield, come to a rapid halt. As a result, the person's body experiences an _unnecessarily large acceleration_ (and thus a large force) over a _short time interval_ as it collides with the now stationary car's interior.

Because of _Newton's First Law_, without the restraint of a seatbelt, the body continues moving forward at 55 mph until it encounters the windshield, dashboard, steering wheel, or some other interior surface. When it hits any of these objects, it comes to a sudden stop over a very short time. For example, if a person is traveling 55 mph in a car that hits a tree and suddenly comes to a stop in 0.10 s, the person's body experiences a force on the order of:

$$ \Delta p= mv_f-mv_i =(75\text{ kg}) (25\text{ m/s}-0\text{ m/s})= 1875 \text{ kg m/s} $$

$$F= \frac{\Delta p}{\Delta t} =\frac{1875 \text{ kg m/s}}{0.10\text{ s}} =1.88 \times 10^4\text{ N} $$

The large force results in a _severe or even deadly injury_. It is much better to be held by the seat belt and decelerate with the car over a distance of about a meter. Then the acceleration during the collision will be on the order of:

$$\Delta p=mv_f-mv_i = 1875\text{ kg m/s}$$

$$F= \frac{\Delta p}{\Delta t} = \frac{1875\text{ kg m/s}}{1.0\text{ s}}=1.88\times 10^3 \text{ N}$$

The deceleration is still significant but at least survivable.