Impact Force Calculator

Impact force is the average force experienced during a collision or sudden stop. It depends on how fast the object was moving, how massive it is, and most critically of all on how long the stopping takes.

Method 1: From impulse and time F = m x Δv / t

Where Δv is the change in velocity (usually from v to 0 for a full stop) and t is the duration of the collision. A car airbag extends the stopping time from milliseconds to about 50ms, reducing peak force on the occupant by roughly 3-5x compared to hitting the steering wheel directly.

Method 2: From kinetic energy and stopping distance F = m x v² / (2d)

This is the work-energy approach: the kinetic energy (½mv²) must be absorbed by the force over the stopping distance d.

Both methods give the average force. A real impact is nothing like constant: force spikes hard at first contact and decays from there. The average is what gets quoted because the peak is difficult to measure and swings wildly with the material.

Practical reference points. A 1 meter fall gets you to 4.43 m/s, since v = √(2gh), and that is the speed all three of the first figures start from:

• A 70 kg person landing on a hard floor, stopping in 0.01 seconds: about 31,000 N, or roughly 3.2 tonnes-force
• The same fall onto a crash mat, stopping in 0.1 seconds: about 3,100 N. Ten times the stopping time, one tenth the force
• A car at 60 mph (26.8 m/s) brought to rest in 0.15 seconds: about 18g on the occupants
• A smartphone dropped 1 meter onto a corner stops in well under a millisecond, which is how peak forces there reach hundreds of g

That second line is the whole argument for padding, crumple zones, helmets and airbags. None of them reduce the momentum you have to shed. They just spread the shedding over more time.