Momentum=mass*velocity
Change in momentum=force*deltaT, where delta T is the change in time associated with the change in momentum.
Therefore, m2v2m1v1=Fdt, where m2=m1 is the mass of the guy in the car, v2=0 is the final velocity of the car and v1=80 mph is the initial speed of the car. We can get the force experienced by the guy in the car as a function of the time it takes for him to decelerate by rearranging the equation relating force, time and momentum:
F=m1v1/dT
If the guy's mass is 100 kg, and the speed of the car is 80 mph = 35.8 m/s, then the force (in Newtons) that the guy experiences is
F=3580/dT
If he decelerates in 1 second, the force is 3580 N, or 3.65g based on his mass.
If he decelerates in 0.5 seconds, the force and g level are both doubled to 7160 and 7.30g, respectively. Beyond this level, he is unlikely to survive.
