Crumple zones. A life-saving development in automobile manufacture is the invention of crumple zones, areas of the body and frame of a car deliberately made to collapse in a collision (Fig. P7.64) such that the passenger compartment will not. Introduced in 1955 on the Heckflosse made by Mercedes, crumple zones became mandatory on all cars sold in the United States in 1967. For a head-on collision with a tree at 40 mi/h (about 18 m/s), find the ratio of the average force on a car with a stiff frame (stop time of 0.010 s) to that on a car with a crumple zone (stop time of 0.25 s).

Figure P7.64

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