A deceleration is an acceleration that slows an object down. The resultant force needed is found from force = mass × acceleration, written F = ma, with force in newtons (N), mass in kilograms (kg) and acceleration in m/s2. The acceleration itself is the change in velocity divided by the time taken.
In a normal stop a car might slow from 20 m/s to rest in about 10 s. That is a deceleration of 2 m/s2, so a 70 kg passenger feels a force of only 140 N. In a crash the same change in velocity can happen in about 0.1 s. The deceleration is then 200 m/s2, and the force on the same passenger is 70 × 200 = 14000 N.
A large deceleration is dangerous because it needs a very large force on the people in the vehicle. This force can cause serious injury, for example when the body is thrown against the seat belt, the steering wheel or the windscreen.
The force can be made smaller by making the stopping time longer. Crumple zones collapse in a crash, seat belts stretch slightly and airbags squash, so each one makes the passenger take longer to stop. The change in velocity is the same, but a longer time means a smaller deceleration and a smaller force.