So, why can a properly executed karate kick break a concrete block without fracturing bones [16]? first, bone is a very strong material. its ultimate compressive strength is approximately 40 times larger than concrete. second, contact is made with the edge of the foot. this concentrates the force into a small area of the target and reduces the likelihood of bending a bone to the point of fracture. third, the collision with the target is essentially inelastic and extends over several milliseconds, so the peak force, though large, does not exceed the strength of the bone. [8] george b. benedek and felix m. h. villars, physics with illustrative examples from medicine and biology, vol. 1. (menlo park: addison-wesley publishing co., 1974). [16] s. r. wilk, r. e. mcnair, and m. s. feld, am. j. phys. 51, 783 (1983). if a (cross-sectional area of the tibia) ~ 2.5 cm2 c m 2 , compute how far a 67 kg person can fall and land stiff-legged on both legs without breaking a bone. assume f is split evenly between two legs. (hint: f≤2σ  a f ≤ 2 σ   a )