4. The normal force of the ground on the foot can reach three times a runner’s b

Question

4. The normal force of the ground on the foot can reach three times a runner’s body weight when the foot strikes the pavement. The femur can be modeled as a tube of cortical bone (Y = 16 x 109 N/m2 ). The cross-sectional area of the bone of the femur can be taken as 5.2 x 10-4 m2 .

a. What is the weight of an 80 kg runner?

b. What is the stress on the persons’femur at rest?

c. By what amount does the 52 cm long femur of an 80kg runner compress at the moment the leg strikes the ground during a run?

#2 I have got 6.9 kg For #4 I have got 0.1304 mm I am not certain of the answers though. I will greatly appreciate your help

Explanation / Answer

a)   weight of an 80 kg runner = 80 * 9.8 = 784 N

b)    stress on the persons’femur at rest = (3 * 784)/(5.2 * 10-4)

                                                                =   4.523 * 106 N/m2

c)   amount does the 52 cm long femur compress = (3 * 784 * 0.52)/(5.2 * 10-4 * 16 x 109)

                                                                                =   0.147 mm

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