A change in momentum needs an impulse, force multiplied by the time it acts. The impulse is fixed by the crash: stopping dead removes the incoming momentum, while bouncing back removes it and then adds the same again the other way, so a rebound needs a bigger impulse. Since impulse equals force times time, stretching the contact time with a crumple zone lowers the force. Keep the peak force under the limit.
MissionKeep the peak force under 120 N. Lengthen the contact time to soften the blow.Round 1Streak 0Best 0
Compared with stopping dead, a rebound needs:
change in momentum (impulse)10 kg m/s
contact time0.20 s
average force F = impulse ÷ time50 N
F t = change in momentum, so a longer contact time means a smaller force.