Finding velocity from g-force?

[FredGirl13]

Okay so this is more of a theoretical question. If say a car crashed into a stationary truck and the driver experienced around 20-25g, what other factors would you need to find how fast the car was going before impact and how would you do that?

 

[pergradus]

A lot, I think the situation you describe is far too complex. Remember that F = dp/dt, and so ultimately it depends on how quickly the momentum the person carries is transferred to the car, and the rock. The car is built to have "crumple zones", there may be airbags involved, the person is strapped into a seat belt, and pieces of the car are probably flying off in random directions.

You'd have to greatly simplify things to do a manageable calculation.

 

[sgb27]

If you know the acceleration of the person somehow at each time instant you can figure out the initial speed by integration. However I suspect the 20-25g figure you quoted is just the peak acceleration, and you would have no way of estimating the shape of the acceleration curve, which means you can't get a very good estimate. If you could find some accelerometer data from similar crash tests (ie same type of car and same accident type) then you might be able to get some kind of estimate, but without such data you'd just be guessing.