Question

When landing poorly from a jump, your leg is likely to break at a point just above the ankle where the cross-sectional area of the bone (the tibia) is smallest. The tibia has a radius of 0.7cm. A bone will fracture when the compressive stress on it exceeds 130MPa. Your intrepid physics instructor (mass 60kg) jumps from a desktop of height 1.2m onto a concrete floor, landing evenly on two legs. How much should she bend her knees to avoid breaking bones? Specifically, what is the minimum stopping distance dd such that the stress on the tibia during her landing is sufficiently low that it will not break? Give your answer in cm. Notice that landing with stiff knees greatly decreases this stopping distance and is thus a really bad idea.

Answer 1