Question

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


)

Answer 1

A properly executed karate kick can break a concrete block without fracturing bones due to the strength of bones, the concentrated force on the target, and the inelastic collision with the target. To calculate how far a person can fall and land stiff-legged without breaking a bone, we use the formula f ≤ 2σa, where f is the force, σ is the stress, and a is the cross-sectional area of the tibia.

Step-by-step explanation:

A properly executed karate kick can break a concrete block without fracturing bones due to several factors. First, bones are very strong, with an ultimate compressive strength approximately 40 times larger than concrete. Second, the contact is made with the edge of the foot, which concentrates the force into a small area of the target and reduces the likelihood of fracturing a bone. 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.

To compute how far a 67 kg person can fall and land stiff-legged on both legs without breaking a bone, we need to use the formula f ≤ 2σa, where f is the force, σ is the stress, and a is the cross-sectional area of the tibia. In this case, a = 2.5 cm². Assuming the force is split evenly between the two legs, we can calculate the maximum force each leg can withstand using f = 2σa. Finally, the distance the person can fall is calculated by dividing the maximum force by the weight of the person.

Answer 2

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