3unitz said:
Fs does not equal the cars kinetic energy - s is displacement and varies among different materials. if youre saying Fs = K then youre implying you can calculate K from nothing but the cars impact force and displacement of the material. since different materials have different strengths, they will have different displacements for the same impact force.
if i hit a sheet of titanium with a hammer its displacement will be less than if i hit a sheet of aluminum with the same impact force. in other words Fs is largely dependant on the material and does not equal kinetic energy.
i know that the shorter the time of collision the larger the force experienced: F = dp/dt, thats the whole point of crumple zones, airbags, seatbelts etc. i just disagree with Fs = K (notice that here F is the impact force).
I didn't say it was equal to its kinetic energy, I said it was equal to the change in its kinetic energy (in the end its equal to its initial kinetic energy simply because we are assuming the crumple zone is large enough to completely stop the car, ie. final KE is 0) . Also the impact force will be different since the aluminum will not be able to provide the same average impact force during collision as titanium due to it displacing more. Thats the point.
If the impact force was the same between the two materials, then the momentum method would not work either since the change in momentum is constant. That means time would also be constant (Ft = delta(mv) = constant)
All I used was the work-energy principle. You are basically saying this principle is wrong, or I'm applying it incorrectly. If I am applying it incorrectly I would be interested in knowing how I would apply it correctly in this case. That would mean I'm missing some other important force acting on the system.
Also, go to the hyperphysics link i posted and read the second frame, where they say the same thing I did.
