How to Calculate Impact Energy
Impact energy is the kinetic energy an object carries at the instant it strikes a surface — the energy that has to be absorbed, dissipated, or transferred during the collision. For an object of mass m moving at impact velocity v, that energy is E = ½mv². When the object reaches that velocity by falling from a height h under gravity, conservation of energy means the kinetic energy at impact equals the potential energy lost during the fall (mgh), plus any kinetic energy it already had before the drop began.
Finding impact velocity from a fall
For a free fall that starts with an initial downward velocity v₀ and drops a height h, kinematics gives the impact velocity as v = √(v₀² + 2gh), where g = 9.80665 m/s² is standard gravity. If the object simply drops from rest, v₀ = 0 and the formula reduces to v = √(2gh). Substituting this into E = ½mv² shows that impact energy equals mgh for a simple drop — height and impact energy are directly proportional, so doubling the drop height doubles the impact energy.
From energy to impact force
Impact energy alone does not say how hard the hit feels — that depends on how quickly the energy is absorbed. Using the work-energy theorem, the average force during the impact is F = E / d, where d is the distance over which the object decelerates to a stop (crumple depth, cushioning thickness, or penetration distance). The same impact energy spread over a longer stopping distance produces a much smaller average force, which is exactly why padding, crumple zones, and packaging foam work: they extend d to reduce F.