It gets better. Thanks.
Energy absorbed by car equals its kinetic energy (assuming headon collision and that both cars stop after), so work done by average force on the car is
[itex]F_{aver}d = \frac{1}{2}mv^2[/itex]
where d is the deformation of the car.
From 2nd Newton's law the average force is equal to
[itex]F_{aver} = m\frac{\Delta v}{\Delta t}[/itex]
Are these 2 average forces the same?
