When designing an energy absorbing system, start by defining the kinetic energy (KE) before impact.
KE = 1/2*mass*(impact_velocity)^2
What is the mass of the moving object? What is the impact velocity?
At the end of the event, do you want the moving object to elastically rebound off the structure with the same velocity it came in with, or do you want the structure to crumple/crush to absorb all the KE so that the moving mass comes to rest? Or you can have some rebound and some crumpling/crushing.
Depending on your answer to that question, there are different designs of energy absorbing systems.
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