How this calculator works
Work done by the stopping force removes kinetic energy. Dividing energy by stopping distance gives the average opposing force. A constant force also fixes the deceleration and stopping time. Use occupant mass with occupant stopping distance, or vehicle mass with vehicle crush and motion data; mixing them gives misleading results. Seat belts and airbags alter the motion history, so no fixed belt distance is assumed.
Formula
E = mv²/2; Faverage = E/d; a = Faverage/m; t = 2d/v; equivalent mass = Faverage/g0, with g0 = 9.80665 m/s².
Worked example
An 80 kg mass stopping from 10 m/s over 0.5 m loses 4000 J. Average force is 8000 N, deceleration is 100 m/s² and stopping time is 0.1 s.
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Frequently asked questions
Is this the peak crash force?
No. The peak depends on the actual force-time history and may greatly exceed the average.
Does this estimate injuries?
No. Injury depends on restraint, force distribution, body region, duration and many other factors.
Why does stopping distance matter so much?
For fixed initial kinetic energy, doubling the stopping distance halves the average force.